“Feeling depressed for a long time,” Song Yang, known on 40th Road as SiSi, posted. “Coming out to bask in the sunshine.” Todd Heisler/The New York Times

A woman begins to fall. With her long dark hair in a ponytail and her black-and-red scarf loose around her neck, she is plummeting from a fourth-floor balcony, through the neon-charged November night.

Below awaits 40th Road, a gritty street of commerce in the Flushing section of Queens. Chinese restaurants and narrow storefronts, and dim stairwells leading to private transactions. Strivers and dawdlers and passers-by, all oblivious to what is transpiring above.

But before the pavement ends the woman’s descent, a few feet from a restaurant’s glittering Christmas tree, imagine her fall suddenly suspended — her body freeze-framed in midair. If only for a moment.

She toils in the netherworld of Flushing massage parlors, where she goes by the street name of SiSi. A youthful 38, she is in a platonic marriage to a man more than twice her age; harbors fading hopes of American citizenship; and is fond of Heineken, Red Bull and the rotisserie chicken at a Colombian place on Kissena Boulevard. Among her competitors, she is considered territorial and tireless.

It is the Saturday after Thanksgiving, and SiSi is in a shabby building’s top-floor apartment, for which she pays her “boss” a hefty fee. She has returned from a market with provisions. She has tried calling her younger brother in China, but he is asleep. She has been on the phone with friends and clients, unaware that she is in the sights of a 10-member police team working vice.

She heads downstairs to stand at her building’s entrance, a necessity of her job. Soon she is leading a man back upstairs — an officer working undercover — as her closely held cellphone casts a glow about her face. Their awkward conversation in her apartment convinces him that SiSi has broken the law, just as it convinces SiSi that he is a cop. She pushes him out and closes the door, though not to the inevitable. She knows from experience what comes next:

More police. Tromping through the dusky vestibule of her building, across the worn scarlet rug and up the 50 tiled steps. Past the Chinese sign that says if you’re looking for the driving school, you’re in the wrong place. Then right to her door.

The handcuffs. The hurried escort to a police vehicle. The humiliation. Again.

SiSi watches the officers ascend on the video monitor she keeps near the door. Under the fixed gaze of one of those lucky cat figurines perched on a table, its paw raised in a wish of good fortune, she begins to pace.

Now they are pounding on the door and shouting Police! Open up! SiSi rushes to the apartment’s north balcony, with its panoramic view of the street hustle below. Day and night, sun or sleet, this is where she and her sister competitors sing their plaintive song to passing men: Massage? Massage?

On the narrow balcony, barely two feet deep, she keeps a broom, a bucket and a small blue stool. Up she steps — and now she is falling, plunging toward the hard tenth of a New York mile that is 40th Road.

A tenth of a mile. Where Mandarin trumps English and a glance trumps the spoken word. Where sex is sold beside cloudy tanks of fish and crabs. Where seedy quarters controlled by local powers are rented to illicit massage operations, and the police make sporadic sweeps, and immigrant women are arrested again and again, and few in this city take notice.

Gravity prevails.

The undercover officer, his job done, exits the building and turns right — at the very moment that the woman who has just offered him intimacy for money hits the pavement at his feet. A woman known along 40th Road as SiSi, but whose given name was Song Yang.

A tiny street where, day and night, women can be heard calling out: “Massage? Massage?” Todd Heisler/The New York Times

In the Google Maps of the mind, pull back from this tiny street to take in the borough’s 178-square-mile sprawl: a pulsating hive of parkways and boulevards, apartment buildings and single-family homes, two airports, a major league ballpark, remnants of a world’s fair — all bracketed north and south by ocean, river and bay.

A striving borough of comity and contradiction, Queens is both the birthplace of the American president — elected in part on an anti-immigration platform — and home to 2.3 million people, nearly half of them foreign-born. With hundreds of languages spoken here, it may be the most linguistically diverse place on earth.

Every day, airplanes alight at Kennedy International Airport in southeast Queens, their passengers including many immigrants who join the borough’s anonymous, aspirational ranks. They chop the vegetables, wash the dishes, clean the toilets, mow the lawns, drive the hired cars.

And some wind up in the commercial sex trade. Making money for a pimp in an airport motel in South Jamaica. Waiting for the next client in a dingy building along Roosevelt Avenue in Corona. Or, like Song Yang, standing on a Flushing street on a cold November night, hiding behind her cute nickname, calling out to men. Playing her role in a shadow economy that benefited others through the exorbitant rent she paid.

“I hear she was No. 1: young, pretty, and her service was great,” said Michael Chu, a travel agent and community advocate who worked across the street from her on 40th Road. “People just lined up for her.”

For years now, Flushing has been an ever-replenishing repository of immigrants entangled in the underground sex economy. The commonplace raids of illicit massage operations across the country routinely lead to the arrests of women with Flushing addresses.

These parlors disappear and reappear with regularity, undermining the police crackdowns often prompted by neighborhood complaints. The industry’s opaqueness adds to the confusion. Some parlors have legitimate state licenses; some legitimate operations have masseuses making sex-for-money side deals; and some are illegally unlicensed, with no interest at all in addressing someone’s sore neck.

Emotionally manipulated by their bosses, ashamed of what they do, afraid to trust, the women rarely confide in the police or even their lawyers about their circumstances. They might be supporting a family in China, or paying back a smuggling debt, or choosing this more profitable endeavor over, say, restaurant work. No matter the backstory, the police say their collective silence further complicates law-enforcement efforts to build racketeering and trafficking cases against the operators.

But society has become increasingly aware of the complexities and inequities of the commercial sex economy, including a criminal justice system that has tended to target the exploited — often immigrant women and members of the transgender community — while rarely holding accountable their customers and traffickers.

In early 2017, New York’s police commissioner, James O’Neill, announced at a news conference that he would redirect his vice division to address prostitution and sex trafficking. This would include training intended to alter what he called the “law-enforcement mind-set.”

“We’ve already switched much of our emphasis away from prostitutes, and begun focusing much more on the pimps who sell them and the johns who pay for their services,” he said. “Like all crime, we can’t just arrest our way out of this problem.”

Since the establishment of this new “mind-set,” the police have continued to struggle at building criminal cases against the operators. But prostitution arrests in New York City have dropped more than 20 percent in the last year, while the arrests of customers have spiked.

Still, this change in attitude at Police Headquarters in Lower Manhattan had not necessarily crossed the East River to benefit an immigrant now lying on her side, unable to speak, gazing up at a plainclothes officer trying to calm her until an ambulance arrived. Beside a spent cigarette, her blood pooled on the pavement she had so often worked.

By morning, Song Yang would be dead, shattering a tight Chinese family that would never accept the police version of events. Her death would also come to reflect the seemingly intractable nature of policing the sex industry, and cast an unwelcome light on the furtive but ubiquitous business of illicit massage parlors.

In the epic of Queens, this stretch of 40th Road is little more than an asphalt hyphen. But along its short expanse exist worlds within worlds within worlds.

Four stories above the lights and shadows of 40th Road, Song Yang rushed to the balcony. Todd Heisler/The New York Times

Baba, I want to go, I want to go.

I want to go to work, the little girl would say to her parents. I want to pick ginseng. She was a born worker, their Song Yang.

She and her younger brother lived with their parents in a remote village in China’s northeastern province of Liaoning, where they grew crops on land allotted to them by their local village committee. Little Song Yang was especially efficient at harvesting the family’s ginseng crop, her mother, Shi Yumei, recalled. “The more her father praised her, the harder she worked.”

Her father, Song Xigui, eventually found moderate success selling construction-grade sand he bulldozed from a nearby river, and by the 1990s the family had replaced its thatched-roof home with a modern brick house that included two “kang” bed-stoves, heated slate platforms that provided warmth during the severe winters. Still, they continued to work the crops, with Song Yang often responsible for running home to light the stove, cut the vegetables and mind her brother.

As she grew older, she began to collect specimens of the enchanting butterflies zigzagging down by the river, and became meticulous in preserving their fragile iridescence. When friends came for boisterous sleepovers, they would marvel at her book of butterflies, and take turns asking to keep one.

Butterflies became Song Yang’s gift.

At 19, she moved 2,200 miles south to Saipan, the largest of the Northern Mariana Islands, an American commonwealth, where she became one of the thousands of young Chinese women who labored in sweatshops to produce apparel bearing the guilt-absolving label “Made in America.” Sharing a room with five other women in a dormitory, she covered her bottom-bunk bed with a silken cloth curtain, and adorned her small rectangle of privacy with family photographs.

Saipan’s garment industry was shrinking by the early 2000s, and Song Yang left to become a waitress on the island. She married a worldly divorced father named Chau Chuong, an American citizen who had worked for years in New York’s restaurant grind. He was so much older — 67 to her 27 — that her family was slow to accept him.

In 2006, the couple opened a small Vietnamese restaurant on Saipan that became so successful they opened a second place, with 150 tables. He worked the kitchen and she worked the front. “She attracted a lot of friendly customers,” her husband recalled.

Her brother, Song Hai, joined her after his high school graduation, eventually opening a henna tattoo parlor with a friend. When their mother came for a visit, she posed for photographs beside her daughter’s well-stocked restaurant bar, her smile radiating pride.

“We had a real sense of accomplishment,” Mr. Song said in Mandarin.

But a catastrophic earthquake and tsunami struck Japan in 2011, disrupting a main source of tourism to Saipan, as well as the fortunes of Song Yang and Song Hai. The restaurants were sold, the tattoo parlor shuttered.

Photographs from her brother’s wedding in March 2013 capture the last happy times that Song Yang spent with her family. Here she is back home, posing with the bride and groom. Here she is, sharing a restaurant meal with the growing family. Here she is.

Song Yang, left, gathered with family for her brother’s wedding a month before moving to New York. Todd Heisler/The New York Times

A month later, Song Yang joined the hundreds who arrive daily at Kennedy Airport, direct from China. Straight to Flushing she went, like so many before, where she hoped that she and her husband would succeed again as restaurateurs.

But there is the dream of Flushing, and the reality.

With her husband now too old for kitchen work, Song Yang became their sole source of income. A waitressing job failed, as did a short-lived Chinese fast-food venture on Main Street. So she became a home health aide, and took a massage-therapy course in the hope of earning additional money. Then a friend told her of a more lucrative opportunity, to be found along 40th Road.

The understanding of her parents and brother was that Song Yang worked in reflexology. They knew that gifts arrived from New York. That she called regularly for video chats while sitting in a black office chair, sometimes eating a bowl of porridge. That when her nephew was born, she proudly announced on social media that she had become an aunt — a “gugu.”

That she seemed happy, mostly. But there was that time when she refused to video-chat for several days, after which she explained that a man had beaten her about the face. And that other time, when she revealed that a man — a law-enforcement officer, she said — had held a gun to her head while forcing her to perform oral sex. Family members reassured her: She had no choice.

Song Yang told her family last fall that she had booked a flight to China for December, and was looking forward to meeting her nephew for the first time. So far she had connected with him only online, through the popular WeChat app, where her avatar sometimes featured a butterfly.

What kind of present do you want Gugu to bring home? she would ask the child, her image beamed halfway around the world from some exotic place called Flushing.

A one-block obstacle course of idling delivery trucks and construction equipment. Todd Heisler/The New York Times

The thoroughfare known as 40th Road was Grove Street, once.

In the 19th century it had a volunteer firehouse, a nursery and residents with Irish surnames. Just in living memory, there was Harry Barlow’s auto garage, the mimeograph services of Case the Printer, an appliance store proud of its color Zenith television sets and, of course, the Old Roma restaurant — famous for its veal cutlets on linguine, and that yellow sponge cake with pineapple filling.

It is all long gone, replaced by ginger duck rice casserole and a shaved ice treat called red bean baobing. The 40th Road of today is almost entirely Chinese, its restaurant signs often featuring no English at all — one more reminder that the only New York constant is change.

The street’s 20 buildings, including Song Yang’s, are mostly three- and four-story structures from the 1980s and ’90s that evoke a utilitarian, Soviet-bloc drabness. Narrow and claustrophobic, they loom like set pieces for a film noir.

The one-way street itself always feels like a wrong turn, an obstacle course of idling delivery trucks and construction equipment. One end elbows past a small playground; the other runs into the ever-clogged Main Street intersection, where plainclothes police officers can often be seen sitting in an unmarked vehicle in an attempt to deter quality-of-life crimes. Pickpocketing is so prevalent that a nearby grocery displays signs of a stick figure reaching into another stick figure’s handbag.

Above, looming airliners grumble as they approach LaGuardia Airport, across Flushing Bay. Just behind 40th Road, Long Island Rail Road trains grind and whine along the raised tracks. Up and down the block, the earthy aromas of produce stands and restaurant waste commingle with the classical Chinese instrumental music emanating from a soup-dumpling restaurant.

And here, beside the upturned fruit crates and the overloaded garbage bags, stand the women of the massage parlors. In their 40s and 50s, mostly, they check their cellphones, drag on untaxed Korean cigarettes bought in bulk, and chat, but with eyes scanning for unattached men lacking a law-enforcement vibe.

Massage?

Aromas from produce stands and restaurants commingle with strains of classical Chinese music. Todd Heisler/The New York Times

The offer is understood, if not explicit. If the man consents, he is led up the stairs of one of the dull buildings, where massage operations are often crammed amid barbershops, driving schools and employment agencies.

Massage parlors offering sex are hardly a recent phenomenon, and business models vary. But the trade along 40th Road is especially audacious. The women stand on both sides of the street — five, 10, a dozen at a time — as ubiquitous as the delivery trucks. In the merciless heat and cold, they sweat and shiver on staked ground, prompting resentful neighborhood complaints about lost business and children exposed to the seamy daily spectacle.

A common arrangement on 40th Road is one in which a “boss” rents an apartment or office from one of the building’s tenants, then provides space to women for a $20 cut of whatever they charge each client. The general expectation is that each woman will generate at least $100 a day for the boss.

But the bosses provide no meaningful protection. The women are at the mercy of the street, where they have been robbed, beaten, raped, thrown down stairs. The surveillance cameras nearly always present are intended less for security, perhaps, than to provide the boss with a way to count the clients who walk through the door.

Over several months, the women along 40th Road shared in Mandarin the stories of how they came to be standing here, offering sex to strangers. They use names like masks. Some have chosen Americanized names — Jenny, for example — while others have been rechristened by bosses with nicknames that sound like Lala, or Kiki, or Yoyo.

They came from all over China, and from myriad backgrounds. One woman said she used to clean houses. Another said she was a former reporter who covered Chinese real estate. Several described the circumstances that left them in economic straits: a failed bus company, a bankrupt jade dealership, a gambling-addicted husband.

One woman often positions herself near a standpipe at the corner of Main Street, so as to be the first to approach any man venturing west. She is in her 60s, small-framed and usually dressed in layers, with long hair dyed black. She said in a raspy voice that she was from the southeastern Chinese province of Jiangxi, and that she was trying to pay off a debt incurred by her adult son in a business deal gone wrong.

She had visited two job agencies on 40th Road, looking for work as a nanny, but nothing panned out. And now she was here, on the corner, where her half-joking refrain — “I’m too old” — did not seem to deter clients.

Another woman, who gave her name as Xiao Li — or Little Li — said she was from the city of Dexing, in Jiangxi Province, home to a well-known copper mine, where she once was a welder. Thin and often wearing a simple black dress, she said she had briefly left the street to study legitimate massage — “So my heart could have a little bit of peace” — but had concluded that the classes were a waste of money. Back she came to 40th Road.

“My body can’t take it,” Ms. Li, 50, said. “My body can’t take so many men.”

Others were even more expansive, including a stocky, 40-ish woman with cropped black hair and a lazy eye who called herself Rachel. Eating a sweet baked potato at a dumpling stall on Main Street, she recalled that while working at a job she loathed — waitressing at a Chinese restaurant in Seattle — she began hunting through WeChat forums for leads on other work, and came across an offer that she recalled saying:

Massage Woman to Stand on Street. $20,000 a month. Flushing, N.Y.

Rachel called the number to ask what the job entailed. The boss replied: Everything.

After her first day, Rachel said, “I got home and took a shower, and cried.”

She paused at the memory, and added, “But then I just thought to myself, ‘I have to keep thinking positively.’”

Michael Chu, the longtime neighborhood advocate, has befriended some of the women who stand outside his building on 40th Road, and occasionally offers them assistance with police matters. His office, where an old dog named Scout is usually napping on some cardboard, is furnished with desks left behind by an accountant who moved rather than work beside a massage parlor.

A bespectacled man of 65, Mr. Chu has listened to the travails of these women, whom he calls “sisters.” The beatings, the robberies, the harassment from teenagers in the playground, the pressure to attract enough clients to cover their “rent” to the boss. The hopes they harbor for permanent residence, for having enough money, for finally not doing this.

“They also have an American dream,” Mr. Chu said. “The sisters have an American dream.”

After Song Yang’s death, her brother would post fliers pleading for information. Todd Heisler/The New York Times

The man spotted her on the street one night after stopping at a 40th Road restaurant known for its cheap and plentiful food. She was pretty, younger than the other women and conversant in English, so he paid for a session. She said her name was SiSi.

His was Paul Hayes. Single, in his early 40s and living in Queens, he carried himself with a seen-it-all air — but she beguiled him. They gradually became lovers, then good friends with vague plans to rekindle their romance someday. But she lived with her husband in an apartment a block away. It was complicated.

She had a good sense of humor, and often solicited his advice — although she ignored him when he recommended bolstering the building’s security system. She also confided about the dangers and vagaries of her work life.

“She really hated doing it,” Mr. Hayes said.

Even so, Song Yang established herself as a fierce competitor in the circumscribed world of 40th Road. Fueled by coffee and Red Bull, she toiled nearly nonstop, as if facing some self-imposed deadline. Word was that she was trying to save up to open her own Vietnamese restaurant, or to buy a house in New York for her aging parents, or to just move on.

Her sharp elbows and inexhaustible style irked some of the other women, leading to arguments, shoves and occasional hair pulling. One competitor recalled that if a man chose another masseuse, Song Yang would tease the client about preferring older women.

But another woman remembered a gentler, more generous Song Yang. She said that when she arrived at 40th Road, Song Yang insisted that she accept several pairs of pants to ward against winter.

Song Yang’s domain was a fourth-floor apartment at 135-32 40th Road, directly above another massage operation. The apartment door faced a boiler room and a makeshift gate that was intended to keep vagrants from sleeping on the roof, but also to protect the hot pepper plants nurtured there by the aged custodian.

As were most things on 40th Road, her rental arrangement was convoluted.

The building was constructed in 1992 by Jentai Tsai, 85, a prominent, even revered, banker in Flushing, and is owned by a real estate company overseen by his son, Eugene Morimoto Tsai. In a brief conversation last month, the younger Tsai, 42, said that he did not know that a woman had fallen from his building last year, or that his building had long been a hub of illicit massage activity.

They both said, and city records confirm, that the building’s managing agent — responsible for collecting rent — was another man of local distinction: Peter Tu, 62, the longtime director of the Flushing Chinese Business Association, a member of Community Board 7 and a district leader for the Democratic Party.

Outside his office around the corner, Mr. Tu at first denied that he was involved with the 40th Road building, but then said that he had merely tried to assist the Tsai family by collecting the $18,500 monthly rent from the main, first-floor tenant, the Shi Li Xiang seafood restaurant. He said he no longer served in this role, had never taken any payment “from the street,” and had no idea what arrangements the restaurant had with the tenants and subtenants upstairs.

“I’m always in the middle,” Mr. Tu said.

A man identifying himself as the boss of the first-floor restaurant began to shout when asked about the tenants upstairs. “How am I supposed to know the names of the people to whom I rent?” he asked in Mandarin. “You want me to go up and ask everyone who they are?”

Above the restaurant, in this building owned and managed by Flushing men of stature, Song Yang paid a flat fee for her apartment — as much as $400 a night, competitors say — to a square-headed, elusive “boss” who goes by Lao Li, or Old Li, a kind of avuncular nickname conveying familiarity with the women who work for him. But the particulars of his subleasing arrangements are as difficult to pin down as he is.

One spring midnight, Lao Li made a rare appearance on 40th Road to mediate a dispute over clients that had erupted among the women. When a reporter approached and called him by name, Lao Li looked up — and bolted. He dashed east down the center of 40th Road, dodging cars, before vanishing into the dark Flushing night.

Although Song Yang and other women often quarreled, they occasionally gathered with Lao Li at the restaurant downstairs, or at a nearby karaoke bar. They’d watch him blow out a candle on his birthday cake, or sing along to a song popular in his native northeast China. At the Chinese New Year, he would hand out red envelopes containing small cash gifts.

In cellphone videos and photographs of these get-togethers, the participants could easily be mistaken for co-workers at an accounting firm, making a night of it. They seemed untroubled by their profession’s many perils, including robbery, bodily harm — and, especially, arrest.

Arrest attracted unwelcome attention. It jeopardized applications for permanent residency. It magnified the humiliation. And it usually meant an appearance at the Human Trafficking Intervention Court, held on Fridays in the basement of the Queens Criminal Court in Kew Gardens, where Mandarin sometimes seems as common as English.

Established nearly 15 years ago, the court set out to treat women in the commercial sex trade less as accused criminals than as victims of trafficking and exploitation. They are told that charges will be dismissed and records sealed if they complete several individualized counseling sessions — focused, say, on job training, or education — with Garden of Hope, Restore NYC, Womankind or another outreach organization. A group called Sanctuary for Families is also on hand to provide immigration services.

Song Yang went through this process more than once. In addition to expunging the arrests from her record, these court appearances provided pause, forcing her to confront the consequences of her work life.

Song Yang’s mother and brother would cling to the possessions she left behind. Todd Heisler/The New York Times

In the summer of 2016, Song Yang began frequent WeChat dialogues with a Flushing lawyer, Chen Mingli, that at first focused on acquiring permanent residency — a process that he repeatedly told her could take months and months. Still, she fretted that her arrest history would thwart her application for a green card.

I am having a lot of anxiety, she wrote in Chinese.

Gradually, though, their conversations came to reflect the darker realities of her 40th Road realm, with sobbing emoticons peppering her messages.

Good morning, Lawyer Chen, she wrote in mid-October 2016. A police officer put a gun to my head today and forced me to perform oral sex.

At the insistence of a friend, she had filed a complaint with the 109th Precinct. Investigators spent the day in her “shop,” looking for evidence and checking the building’s surveillance video, which had captured a heavyset bald man in a suit ascending the stairs.

Mr. Chen assured her that the matter would not affect the status of her immigration case, and implored her to cooperate with the police. But her intense desire to avoid attention, coupled with fear of retaliation from her attacker, overshadowed everything.

The police said that this won’t affect me in any way, but I’m afraid that it will … Lawyer Chen, what am I going to do now? … (Sob)(Sob)(Sob)

The police circulated a wanted poster based on a hazy photograph of the man lifted from the surveillance video. A retired United States Marshal, who surrendered after someone mentioned him as a possible suspect, participated in a lineup.

But Song Yang identified another man, wrongly, as her attacker. In addition, a DNA sample from the retired marshal did not match samples taken from Song Yang’s clothing. The case was eventually closed.

Several months later, in late September 2017, she was arrested a third time on a prostitution charge. Handcuffed, led away from 40th Road, held overnight.

A few days later, Mr. Chen asked, You’ve been arrested again?

Song Yang answered:

Yeah. (Sob)(Sob)(Sob)

She explained that she had been forced to make hard decisions and that it had been difficult to suppress her feelings while married to a much older man who seemed increasingly removed from her day-to-day life. She felt “morally depraved,” and sometimes thought about giving it all up and going home — or worse.

I’ve been having thoughts of jumping from a building, but what should I do? she wrote early one morning.

Mr. Chen was never formally hired by Song Yang, but now his central role seemed to be to buoy her spirits.

Don’t be scared, he wrote hours later. Don’t think that way.

Song Yang only sank deeper.

I’ve fallen so low I can’t be saved.

She continued:

Without purpose, without direction, what meaning is there to keep on living?

I used to be a woman who was very strong in her life. I strove for perfection in everything I did. I never thought that my life would turn out this way. I’ve truly failed.

At the end of October, Song Yang made one last visit to Mr. Chen’s office. She confided that another client had badly beaten her a couple of weeks earlier — an assault she had not reported to the police — and showed him photographs of her bruised and swollen face.

“Why am I so unlucky?” he remembers her asking.

The tiny street is almost entirely Chinese, its restaurant signs often featuring no English at all. Todd Heisler/The New York Times

The case began with an anonymous complaint: Several women were said to be “selling intimacy” at the building at 135-32 40th Road.

The tip hardly came as a revelation, since shady activities at this address had generated scores of 911 calls over the years. To some, the building even had the aura of being cursed, following a horrific crime in 2010, in which a deranged stalker stabbed a woman in the second-floor hallway and removed her heart and lungs.

Forty-three arrests had taken place in the building over the last decade, more than a few sex-related, the most recent that of Song Yang. Ensnared in an undercover sting in late September, she had tried but failed to hide in the cramped boiler room across from her apartment, and was charged with offering sex for $70.

Her case, which had prompted those despairing messages to the lawyer Mr. Chen, was one of 91 massage-parlor-related arrests in the 109th Precinct in 2017, and one of six along 40th Road. According to court records, none of those arrests were for pimping, solicitation or operating an unlicensed massage parlor.

A few nights after the anonymous complaint, a sergeant and a detective ended a brief surveillance by venturing into the notorious building. The only thing they found suspicious was a handwritten sign in Chinese on the second floor, which they believed to say, in effect, There are no girls on this floor; please go to the third floor.

The police later determined that the sign actually said, “Attention, the driving school is on the third floor next door.”

An undercover officer then telephoned a woman associated with the building who was known as SiSi. They arranged an appointment for the next evening, Saturday, Nov. 25. Her price: $120.

On the appointed day, members of the Queens North Vice Enforcement Squad met at their base in College Point to discuss the seven locations they planned to hit that night. The closest target became the first: the bleak building at 135-32 40th Road.

The vice officers went over their safety plan. They chose their identifying color of the day. They agreed upon the mission’s assorted distress signals and code words, including what the primary undercover officer would say to indicate that sex had been offered for money. Now they were ready.

The 10-member team headed out into the evening, unseasonably mild for late November. They parked along Prince Street, across from the White Bear dumpling place and just short of where the one-way street bends east to become 40th Road. The team leader and two arresting officers sat in the first car, with two more arresting officers in the second car. The third vehicle was for prisoner transport.

The team tested its recording device, which used Bluetooth to transmit one-way audio. No problem. The green light was given: Go.

Minutes later, the undercover officer approached his target, Song Yang, just inside her building’s entrance. He wore an olive-green jacket, jeans and a cap. She wore a short winter coat, a red-and-black scarf, leggings and one of her signature headbands — with a small bow that resembled a butterfly.

The officer could not have known that this woman had just attempted a video chat with her younger brother, who was still asleep in China. That she had plans to fly home in December. That she had kept her court-mandated appointments with Restore NYC, a nonprofit organization that helps foreign-born victims of sex trafficking. That her fifth and last session with Restore was four days away.

About all he knew was her police nickname for the night: “JD Ponytail.”

Jane Doe Ponytail.

She led him up the worn stairs. She gave him a peck of a kiss in the hall, and opened her apartment door. Another woman, brand-new to Flushing and known as Momo, was already occupied with a man in the second bedroom.

Song Yang walked her client to her bedroom, where, according to the police, she offered sexual intercourse for the reduced price of $80. He consented to the arrangement and, heading to the bathroom, managed to utter the code word into his transmitter that a positive — that is, illegal — agreement had been reached. He also hoped to signal to colleagues that it was time to move in, but a wary Song Yang prevented him from having privacy, telling him to keep the bathroom door open.

“This is bad service,” the officer said.

Once in the bedroom, Song Yang became even more suspicious. Why aren’t you taking off your clothes? she asked. Are you a cop?

No, he answered. But he complained again about the service and grabbed his hat, prepared to leave. She pushed him out and closed the door.

Responding to the undercover officer’s signal, the three idling police vehicles turned onto 40th Road, smack into its everlasting gridlock. Four officers got out and hustled to the building. Climbing the dreary stairs, they passed their undercover colleague, who pointed to Song Yang’s door as he descended — and as she watched on the monitor in her apartment.

With the police demanding that she open the door, and preparing to break it down, a panicked Song Yang hurried to the apartment’s north balcony. The other woman, Momo, emerged naked from her bedroom to investigate the noise, but hustled back to hide when she realized it was the police.

Surveillance cameras captured a fateful encounter.

The balcony was not equipped with surveillance cameras, leaving what happened next to the imagination. It is possible that Song Yang was hoping to escape, perhaps by reaching for a wire that ran vertically past her balcony. It is possible that she was trying to land on the protruding metal sign of the restaurant below. It is also possible that she intended to kill herself.

It is fact that she hit the pavement directly in front of the undercover officer she had pecked on the cheek just five minutes earlier. His supervisors say that the officer remains shaken to this day.

Later that night, while Song Yang was lying in a hospital bed with multiple fractures to her face, head and body, the police placed her under arrest. She died in the morning — and the arrest was, in the parlance of the police, “voided.”

Where Song Yang once lingered, the terms of transactions were understood, if not explicit. Todd Heisler/The New York Times

In the dark of an early December morning, two weary travelers shuffled through the multicultural scrum of Kennedy Airport. One was a tall, reedy man named Song Hai; the other, a slight, older woman named Shi Yumei, whose protracted weeping on the long flight from Beijing had concerned an attendant.

Song Yang’s mother and brother had traveled 7,000 miles to better understand the how and the why of her death.

A telephone call from her husband several nights earlier had disrupted everything. Song Yang is dead, he had said. Police say she jumped from a building.

Her distraught parents had telephoned their other child, Song Hai, to deliver words so heavy that he dropped his smartphone, cracking its glass. Not accepting what he heard, he sent a WeChat message to his sister that depicted a pair of clinking coffee mugs, along with a gentle request to please call home.

The lack of an answer was the answer.

The mother and brother spent their first two weeks in Flushing tending to the affairs of death. Then, on a dismal day of late December rain, they made their way to the Chun Fook funeral home, a few blocks from 40th Road. Though some had recommended a modest ceremony, the family had insisted on a more elaborate service, in a spacious room with a chandelier.

The dark wood coffin sat at the front before rows of chairs that would remain empty. No women from 40th Road. No Lao Li. A pair of vertical scrolls with parallel aphorisms written in Chinese calligraphy — “Put Down Your Burdens and Return to the Lord” and “Take Up Tranquillity and Celebrate Everlasting Life” — hung on either side.

One minister delivered prayers in English, while another repeated those prayers in Mandarin. The few mourners included Song Yang’s close friend Paul Hayes; the community advocate Michael Chu; Chen Mingli, the lawyer who had tried to help her seek permanent residency; her husband, Chau Chuong, now 78, who had come from California, where he had been living for his health; and her mother and brother, their heads bowed and hands folded.

The ceremony ended with the reading from the Book of Common Prayer that we are all from dust, and to dust we shall return. Alleluia, the mourners mumbled. Alleluia.

Song Yang’s funeral service drew few mourners, and none of the women from 40th Road. Todd Heisler/The New York Times

Then it was a short drive along the Grand Central Parkway to the All Souls Chapel and Crematory at St. Michael’s Cemetery. This is where Song Yang’s battered body was returned to dust, and where, in his frustration and grief, her brother vowed justice and punched a wall.

The official explanation for his only sibling’s death made no sense to Mr. Song. After all, she had already paid for her flight home to celebrate their mother’s upcoming birthday and to meet, for the first time, his 5-year-old son. Suicide was not possible, he reasoned. Darker forces might be at play. He had already begun his own investigation.

One snowy night soon after arriving from China, Mr. Song appeared at his sister’s 40th Road building with Mr. Hayes. Their plan was to break into her apartment, collect her belongings — and, if possible, retrieve any surveillance video.

Mr. Song, a learning specialist by trade, and Mr. Hayes, a computer consultant, crept up the 50 tiled steps to the fourth-floor door, which was secured with a locked chain. Fearing the noise of the hammer and small acetylene torch they had planned to use, Mr. Hayes hustled to a Home Depot a mile away and returned with a hacksaw.

After a few minutes of sawing, the chain gave way, and the two men pushed open the dull-gray door to enter the setting of a life interrupted. The police had taken the surveillance equipment, but everything else made it seem as if Song Yang might return at any minute.

In the two bedrooms, rumpled sheets. In the kitchen, a Pepsi and a half-empty bottle of Bacardi, sliced carrots and apples, and the black chair that Mr. Song recognized as the one his sister sat in while video-chatting with her family. In the living room, a raised table with a red curtained skirt, on which sat a CD player, a pair of sunglasses and a lucky cat figurine. Placed neatly on the floor, a pair of pink shoes.

On the snow-dusted front balcony, a broom, an upside-down bucket, a stool, a few plastic bags containing fruit and eggs. And, just beyond, the beckoning lights and shadows of the street below.

Song Hai traveled from China to investigate the circumstances of his sister’s death. Todd Heisler/The New York Times

Song Hai returned often to 40th Road, a spectral presence in his dark hooded coat and black cap, a cigarette cupped in his hand. He cajoled and confronted the sidewalk’s denizens, asking questions, taking photographs, recording conversations. He saw himself as a lone-wolf investigator, working to prove that corrupt officers of this strange city had thrown Song Yang over the railing.

His ever-evolving theory:

That his sister had been sexually assaulted by a police officer. That she had filed a complaint. That the subsequent police lineup was fixed to protect the assailant. Then it was payback, which explained why, of all the women along 40th Road, only Song Yang was arrested in September, and was about to be arrested again in late November.

As is standard when a death occurs during a police action, the Queens district attorney and the police department’s Force Investigation Division were investigating. But Mr. Song was already beginning to believe that nearly every corner of the American criminal justice system — from the police to the medical examiner — was colluding to hide the truth.

He patrolled downtown Flushing. He interrogated women and shopkeepers. He plastered the streets with leaflets featuring photographs of his sister and promising a “Big Reward!” The plea appeared in Chinese and in fractured English:

Hello! When you saw the photo, SiSi (Song Yang) was no longer alive. She fell from and died on 11/25/2017 at 135-32 4FL in Flushing. Families as well as the NYPD Internal Affairs Bureau are eager to find out the truth of her death. If you have ANY CLUES, please contact me ASAP. Absolutely Confidential … (Her brother Song Hai)

The dozens of responses yielded little. One man called to say that Song Hai’s sister was a whore — a word he did not quite understand, and so he continued the conversation: Yes, yes. And do you have information?

His sleuthing occasionally paid off. One evening, amid the Main Street crush, Mr. Song spotted a man he recognized from his sister’s WeChat photographs: short, solidly built, and with a distinctive, block-shaped head.

Excited, Mr. Song crossed the street and, right at the Roosevelt Avenue intersection, near the subway entrance, grabbed the man by the arm. Mr. Song recalled what happened next:

Are you Li? he asked.

You’re mistaken, replied the startled Lao Li, the boss who controlled Song Yang’s apartment. My name is not Li.

Mr. Song waved down a passing police car, as a crowd gathered and the agitated man in his grip implored him not to involve the authorities. Let’s resolve this ourselves.

The two officers understood Mr. Song’s intentions, he later recalled, but they explained that this was America, not China, and that he was unlawfully detaining a man who wasn’t present when his sister’s fall took place. They separated the men, and Lao Li floated away in the rush-hour stream.

Later that evening, an angry Lao Li telephoned Mr. Song. In the conversation that Mr. Song duly recorded on his phone, Lao Li vented about the audacity of summoning the police — “If you don’t have evidence, how could you say I’m the boss?” — before giving his version of the realities on 40th Road.

He said that he rented the apartment to Song Yang for $3,100 a month — hardly the $12,000 that was rumored on the street. “She and I really didn’t have any employment relationship,” he said. “Just that at the start of the month, I would take the rent.”

Lao Li said she called him her “boss” so that others wouldn’t bully her, but he insisted that she was her own boss: prone to arrest, sure, but also smart, tenacious and tough.

Throughout the conversation, Lao Li characteristically remained at a distance — even when describing that fateful night. He rushed to 40th Road after receiving a call that “SiSi had jumped from the building,” he said, but by the time he got there, “your sister had already been taken away.”

As Mr. Song conducted his frustrating investigation, his mother spent her days in the numbing cocoon of grief. Once so proud of her entrepreneurial children, Shi Yumei was now a sorrowful woman in a foreign world, gingerly navigating a small cart down crowded Main Street, her pale gray knit cap pulled low, her mind occupied with worry. How, for example, would she and her son, here on temporary visas, survive on the little money they had brought with them?

An encounter with a bellowing street evangelist eventually led her to St. George’s, the old Episcopal church on Main Street whose steeple has long been a Flushing landmark. Its congregation embraced her, smothering her with food, clothing and compassion. Locking hands and forming a prayer circle one day, strangers asked God to grant peace to this new person among them, who felt so blessed that she began to volunteer at the church’s food pantry as a way of giving back.

Grieving her daughter’s death, Shi Yumei found solace volunteering at a local church. Todd Heisler/The New York Times

One cool April morning, she donned an orange apron and joined 40 other volunteers, nearly all of them immigrants, as they prepared for the ritual that unfolds every Wednesday along the old church’s north side, opposite a Lucille Roberts fitness center. They unloaded the crates from trucks, bagged the fruit and vegetables, and established an assembly line of food down the sidewalk: turnips and fennel, lettuce and apples, onions and melons.

The decade-old operation had gradually adapted to the ways of Flushing, with organizers taking note of the tensions caused by different understandings of personal space among ethnic groups. The solution: two alternating lines — one that was entirely Asian, and the other a mix of black, white and Latino.

On this morning, the Asian line ran alongside the church’s cemetery wall, and the other line stretched down to a firehouse. But things moved apace, thanks in large part to the high-spirited efficiency of the volunteers — including Shi Yumei, who smiled at her sense of belonging as she proffered bags of onions.

Each evening, after long days of volunteering and investigating, mother and son returned to a worn apartment catering to transients, not far from 40th Road. Some lodgers paid $20 a night for a narrow bed in the living room. But with some financial help from the church and a few nonprofit organizations, Mr. Song and Ms. Shi managed to pay $1,000 a month for a cramped bedroom and first dibs on a shared kitchen.

They slept beside a closet packed with clothes and accessories that Song Yang left behind. Tears were shed over a single strand of black hair found on a coat. They lived in the presence of her absence.

To find sleep, the mother would hold a small audio device close to her ear and listen to lively recorded stories about historical Chinese triumphs, cuddled beside one of her daughter’s plush teddy bears. This way, Ms. Shi said in Mandarin, “I know my daughter is here with me.”

Two feet away, her son would lie in his twin bed near the window, cigarettes on the nightstand, spent beer cans under the bed, another Long Island Rail Road train clattering in the distance. Here he would try to piece together the stray bits of his investigation.

He had found a grainy photograph from his sister’s WeChat feed of the stocky, bald man who had supposedly sexually assaulted her, and convinced himself that a bald police detective, appearing in cellphone videos taken on the sidewalk after his sister’s fall, was the same man.

He had also obtained photographs and forensic notes from the autopsy. Poring over the graphic images, he decided that the discoloration around his sister’s face came from a beating, and that her broken fingernails suggested some kind of struggle — and, therefore, a cover-up.

This was America. Not China. Exactly.

Song Yang knew that an arrest usually led to an appearance at the human trafficking court. Todd Heisler/The New York Times

On a sunny spring day, those invested in the proceedings of the human trafficking court filed into the basement courtroom in Kew Gardens. Among them were Song Hai, in a black blazer and brown work boots, and Shi Yumei, her blue-and-orange scarf recalling the colors seen on the Staten Island Ferry, on Knicks uniforms, on Mets baseball caps — the colors of New York.

They took their seats among the defendants, including a woman in glasses often seen calling out to men on 40th Road. Mr. Song sat with his hands clasped and back erect; his mother was bent forward, as if in prayer. They waited.

An air of empathy defines the court, which is intended to encourage women engaged in the commercial sex trade to avail themselves of counseling and other diversionary programs. On most Fridays, the judge, the prosecutor and the defense lawyer are women, and the lanky head court officer is determined to make the defendants feel safe and respected. He does his best to dissuade any pimps or bosses from taking a seat.

Prominent in the dozen pews are Chinese women facing the usual massage-parlor-related charges of prostitution or unlicensed massage. Court-appointed lawyers from the Legal Aid Society or Queens Law Associates usually guide them through the process, along with a Mandarin-speaking interpreter and advocates from one of the nonprofit groups specializing in sex-trafficking outreach and immigration services.

Defenders of the program maintain that until a better approach is developed, arrests — followed by appearances in trafficking court — provide the best chance for intervention. Even if a woman returns to 40th Road, they say, she will at least have the names of people to contact if she needs help.

Others, though, counter that for many women caught in the commercial sex trade, an arrest only exacerbates their trauma. Besides, they say, one doesn’t need to be arrested in order to receive helpful contact information.

Judge Toko Serita, who has presided over the trafficking court for a decade, summons the defendants, one by one, to stand before her, as a court officer calls out, “Mandarin interpreter required, and present.” The judge has short black hair, glasses and a welcoming, even reassuring demeanor, whether it’s the defendant’s first appearance or her last.

How are you today? … Are you studying English? … This is a really good streak … I want to congratulate you for completing all your sessions with Garden of Hope … Stay out of trouble, lead a law-abiding life for the next six months, and the record will be expunged … Good luck to you.

On this morning, several cases were heard before a court officer finally called out: “03585 dash 17. Yang Song!”

Even though she was five months dead, Song Yang still had an open criminal case: the arrest on a prostitution charge two months before her fatal fall. The sprawling New York City judicial system may seem overwhelmed, even chaotic, but in the end its books must be balanced. This meant that a formality known as an “abatement by death” — a dismissal, in effect — was required to close the short chapter on Song Yang, or Yang Song, as the system sometimes rendered her name.

Judge Serita was informed that the deceased defendant’s mother and brother were present and would like to thank the court. The request stilled the courtroom. The judge sighed in sympathy.

“Thank you,” she said. “Um. All right. This case is now going to be abated by death.”

She went on to tell Song Yang’s mother and brother that everyone involved in the trafficking court was deeply saddened by their loved one’s “tragic and untimely death.” She expressed hope that they “somehow find peace with these unfortunate circumstances.”

Mr. Song and Ms. Shi acknowledged her words with nods. They walked out of the courtroom, past an “Exit Only — No Re-Entry” sign, and into the late-morning brightness. He lit a cigarette. She adjusted her backpack. They continued on in silence.

Two weeks later, the mother and brother returned to Kew Gardens for a long-awaited meeting with investigators from the Homicide Investigations Bureau of the Queens district attorney’s office. With everyone seated around a dark-wood conference table in a windowless room, the investigators shared the results of their monthslong inquiry, including 22 minutes of video culled from cameras positioned both inside and outside the building at 135-32 40th Road.

In these images, their beloved daughter and sister appears in the fullness of life. Here is Song Yang, leading the undercover officer up the stairs. Here she is, kicking him out of the apartment, watching the officers ascend the stairs — rushing in alarm toward the balcony.

Here, from street level, something falling, and then a beloved daughter and sister, crumpled on the pavement. Watching the video again a few days later, Ms. Shi noticed the headband that flies off her daughter’s head.

“She especially loved butterflies,” the mother said.

The video over, the investigators laid out their sober findings: The police involved in a bust-and-buy sting on 40th Road the night of Nov. 25, 2017, did not cause the death of Song Yang. To begin with, no officer was even in the fourth-floor apartment when she jumped or fell.

Her brother scoffed at this conclusion. He said something rude in Mandarin. Meeting over.

As months passed and Song Hai’s fliers were torn and papered over, the trade on 40th Road endured. Todd Heisler/The New York Times

Night comes to 40th Road.

The fruit and vegetable peddlers have boxed up and boarded up, and the last patrons of air-conditioned restaurants have stepped out to evaporate in the late June warmth. But the women are here, as always, calling out an invitation that sounds almost like a plea.

Massage?

They stand outside the same doorways, including the one for 135-32, where Song Yang once lingered. Very soon after she died, her fourth-floor apartment became the address for a new massage business. Its name: Heaven on Fourth.

A few steps away, at the entrance to another of the gloomy buildings owned and operated by prominent Flushing businessmen, a thin woman in a brown dress sits in a metal chair with a square of Styrofoam for padding, studying her cellphone through the smoke of her cigarette. Then she pulls out a bag of overripe cherries from the building’s broken mailbox and, between the repeated offerings of her services, spits out the pits and tosses them into the street, not far from a lamppost adorned with a poster bearing the face of Song Yang (“Big Reward!”).

To the woman’s right, roasted duck carcasses hang in the window of the Corner 28 restaurant, where a man is mop-swabbing the sticky floor. To her left, sorrowful creatures loll in a seafood restaurant’s murky tank. Above her head, scaffolding provides protection from the stucco that city officials say has been coming loose from the buildings. Rain begins to fall.

A nearby tanker truck groans as it sucks away a restaurant’s used cooking oil through a large hose that snakes across the sidewalk and into the bowels of a building. The women adapt: They step over the hose, ignore the smell, raise their voices.

One of the women leads a potential client to a building’s threshold, but he keeps walking; she mutters an epithet in Mandarin. Then a buzz-cut junkie, who just hours before was asleep on the pavement, begins to harass the women, disrupting their business by hovering, touching, dropping his sweatpants and simulating sex acts. He enters one of their buildings and urinates in the hall.

The rain hardens. The whoosh of a shuttered metal gate resounds. A kitchen worker emerges at the end of his shift and wishes the women a good night. They wish him the same.

It is all ephemeral, of course, a realization reinforced daily by the laborers trudging down this street to the subway, bone-weary from working another of the construction projects that are redefining Flushing. Few today remember the Old Roma restaurant that once thrived on 40th Road, just as few tomorrow will remember a Chinese immigrant who once died on 40th Road.

For now, at least, if you linger on the street, you will encounter those who remember her — including, occasionally, clients still looking for SiSi.

You might see Lala, and Kiki, and Yoyo, along with other women who competed with Song Yang. You might see her lanky brother, Song Hai, who still struggles to understand why no one will be brought to justice for all that his sister went through in her adopted country. His grief smolders, as does his distrust of America.

Lastly, you might see Song Yang’s mother, Shi Yumei.

One evening, Ms. Shi paused outside a building where some women were offering massages to passing men. Raising the drooping bags held in her hands, she explained that she had just left the food pantry at the Episcopal church on Main Street, where she had recently been baptized. She said the pastor had emphasized the importance of sharing what you have.

The mother placed a bag of sweet potatoes in the doorway that had once been Song Yang’s domain. It was an offering of sorts, a gift to women like her daughter. Then she was gone, assumed into the Flushing blur.