In the happy-endings business, it pays to put on a happy face.

Claire knows this well. On a sunny fall morning, she took the train from her home on Long Island to a storefront in Chelsea, where the windows were taped over with yellowing paper.

A maroon sign with white letters provided the only clue about what business was being conducted here: “Spa.”

Inside, it was as dark as a movie theater, the paper and heavy curtains blotting out any sunshine. The smell of sweat rose from the carpet. Soon the place would fill with customers, so Claire changed into a strappy zebra-print dress and steeled herself with a smile for the job of giving massages, and occasionally more, to a parade of men, something she does for 80 hours a week.

Her parlor does not advertise happy endings — that all-too-familiar euphemism — but many clients expect them, she said. Mostly she says no, but sometimes she agrees for those she considers “a nice person,” particularly for those undergoing trouble in their romantic lives.

“They need help, and I help them,” she said.

Though Claire is careful not to discuss a price for this beforehand, she knows that providing a mid-massage “manual release” is likely to garner her a gratuity of $60 per customer, money she says she needs to support herself and her 16-year-old son. Claire could make more — as much as $150 per session — if she went further and had sex with her clients. She refuses.

And while most customers shrug it off if she turns them down, some take offense. One grew furious and slammed her against a wall. She slapped him twice in the face and he bolted, fumbling with his clothes as he ran out. But the encounter left her in tears, shocked at how much her life had changed from her time in China, where she worked for 20 years as an accountant for a respectable business.

(Employees) say it’s just like a 9-to-5 job. Go in the morning, jerk a bunch of dudes off and go home at night. - Andy, a Mandarin translator

“He couldn’t believe it,” she said. “I thought, ‘Why did this happen to me?’ ”

Claire is not her given name, of course. She arrived in New York from Shanghai in 2012 on a work visa, part of an army of Asian workers who support the booming business of illicit massage.

A research study last year by the Urban Institute in Washington, DC, found 4,790 erotic massage parlors on just one site, EroticMP.com, which posts addresses and user reviews. Approximately 1,200 are open just in New York City (which, by way of comparison, has some 2,500 bars and nightclubs, along with 280 or so Starbucks).

If each place clears $20,000 a month, a figure that two Rutgers University professors found was the average among illicit parlors, the industry’s yearly profits would top $1 billion. And this market is trending up. The Department of Labor expects the massage industry to grow by 23 percent between 2012 and 2022.

The industry has become a pipeline for new immigrants, and brought paid-for sex to Main Street America with a veneer of respectability, or at least mystery.

“What explains the growth?” asked James Finckenauer, who co-authored the 2010 Rutgers study with fellow professor Ko-lin Chin. “Erotic massage does get ignored. It’s relatively safe. And setting up a low-level massage parlor doesn’t take a lot of capital. Plus there’s a general interest in society at large in getting legitimate massages. So, crossing over into something more?

“It doesn’t take a lot to bridge that gap.”

The Flushing pipeline

Claire never intended to become a sex worker. The single mom left her country for a better life, starting work in a Manhattan nail salon until she could no longer stand the fumes. A friend offered a crash course in deep-tissue therapy, which led to her new position as an unlicensed masseuse.

The trade is fueled by hopeful immigrants like her — predominantly from China, Korea, Thailand and the Philippines. The Rutgers study found that sex workers in Los Angeles took home, on average, $7,200 a month. In New York the figure was $6,000.

The owners take home much more, and an Urban Institute found that profits are primarily put into other businesses or property. Like a cab license or a bodega, a massage parlor — illicit or otherwise — is seen as a pathway to success in America.

If there’s an unofficial headquarters to the erotic massage industry, it’s the working-class community of Flushing, Queens.

Here, amid discount phone outlets, barbershops and bubble-tea stands, a black-market massage mecca hums. Entire blocks along Union and Main streets are lined with provocative-sounding parlors like Asian Kitty and Shangri-La Lily, and behind the scenes an extensive training network prepares the next generation of pleasure providers.

EroticMP.com lists 29 such establishments in Flushing, though more than that exist, industry sources say.

To be sure, many discourage hanky-panky, offering only typical beauty and health treatments — exfoliation, facials, reflexology, waxing — for women, couples, families. At clubs like Tai Huang, Coco and New York Spa & Sauna, a traditional Korean day spa, one can find a better-than-average shiatsu massage for as little as $25, hot stones included, reviewers say.

The bargains are partly a result of the inexperience of staff members, who are mostly newcomers learning the craft and speak little English. Some go on to get licensed, which in New York state requires 500 hours of training. Many do not bother. There’s a brisk trade in forged credentials.

Andy, a Mandarin translator, learned of the network when he helped a young Chinese couple open their own happy ending parlor. The man had come to New York to attend college but grew interested in the business after meeting his girlfriend, who worked as a masseuse and knew several employees in Flushing.

“That’s just what her and her friends all do,” Andy said. “They say it’s just like a 9-to-5 job. Go in the morning, jerk a bunch of dudes off and go home at night.”

He found the couple a location near Times Square and within weeks the business was up and running, the girlfriend acting as house “mama-san” and her boyfriend handling the books. The clients, Andy said, are mainly bank executives who work at offices nearby. The spa staff consists of women who live in Queens.

“Everyone just comes from Flushing,” he said. “They take the 7 train to Midtown. They work until about 5, and another shift starts at 4 and does the night shift.”

The ‘hobbyists’

One of the lustiest locales, according to reviewers, is Silk Tigers, an upscale Korean parlor on West 38th Street in Midtown. It’s rated No. 3 in the country on Spa Hunters, whose members call themselves “mongers” or “hobbyists” and file detailed reports on the places and providers they like.

Silk Tigers’ online ad suggests more than massage is on the menu: “nude adult relaxation,” “mutual touching” and “erotic body contact,” it offers. “The most beautiful young and friendly girls from Asia and beyond.”

Tabby, a petite brunette with a girlish smile and hourglass figure, is one of the spa’s featured masseuses. Wearing tight-fitting mini-dress, she greeted a reporter on the second floor of a nondescript four-story walk-up near Seventh Avenue.

It’s a narrow, sumptuously appointed space with decor you might find at a boutique hotel: polished wood floors, slate-gray walls with framed mirrors, white leather armchairs, fresh bouquets in cut-glass vases.

“You want massage?” Tabby asked in halting English.

She provided a tour, showing off the shower room, a tiled enclave in which clients are told to lie down on a padded table before being soaped up and rinsed with a hand-held sprayer.

The treatment room had an oversized massage table covered in crisp linens, muted sconce lighting and a small stereo playing chamber music.

She explained the pricing. The house fee of $80 covered the basics: the table shower and hour massage. “And if you want more?” the reporter asked. “What you like?” she said. “How much is everything?” The answer was an extra $140. Presumably, a gratuity would also be required. The reporter declined.

Tabby offered a few personal details. She said she was from Korea and had been in the US for two years. She’s studying to be a hairdresser. But it would seem her life revolves around the parlor, where she works seven days a week for three weeks straight. “Then no work for one week,” she said.

Online reviewers say Tabby and the four other Silk Tigers will engage in just about any illicit act. “It’s a fantastic place,” said one. “They treat you like a god.”

It’s that approach that resonates with men who frequent massage parlors, who covet the attention, the pampering, the feeling of being taken in and taken care of.

A frequent customer named Bill said he enjoys the air of mystique to the spa experience — whether the massage is sensual or strictly therapeutic. Often, he said, he’ll book an appointment not knowing which kind he’ll get.

“The lines have blurred,” he said. “You have to be open to whatever. I’ve been offered happy endings and said no, and I’ve asked and been turned down. There are some really sketchy-looking places in Chinatown where they absolutely will not touch you. And then there was the time I was in Shanghai at a luxury hotel. They had a spa with a stunning therapist who was quite accommodating.”

The will-she-or-won’t-she question represents a sharp departure from the way illicit massage parlors were run in the 1970s, when the women did not pretend to know anything about proper muscle kneading and got right down to business, said Ronald Weitzer, a sociology professor at George Washington University and an expert on prostitution. Today, there’s an element of ambiguity that stems from the explosive growth of spas in Asia.

“Hong Kong is full of massage parlors,” he said. “They do provide legitimate massage. But whether they also offer sexual service depends on the therapists and the customers, some of whom just want a back or foot massage. Sometimes they’ll provide happy endings but only on a very selective basis.”

Hard to police

This ambiguity makes law enforcement difficult.

Owners can easily say they have no idea what goes on in an individual massage room, and they don’t condone sex for money. Inside those rooms, meanwhile . . .

“The cops aren’t supposed to take their pants off,” one NYPD supervisor said. “So that’s a problem. Most of these transactions occur after the massage starts. There’s rarely a clearly stated verbal agreement beforehand, which you need and needs to be recorded if you hope to bring a winnable case. How is the undercover going to get that?”

Even if a case is made, solicitation arrests, particularly for first-time offenders, often lead to wrist-slap charges, which rarely dissuade parlors from reopening. In Queens, for example, the NYPD made 686 arrests for prostitution and loitering in 2014, which included streetwalkers, escorts and erotic masseuses, but all were misdemeanors. Those efforts did little to disrupt the Flushing industry activity.

There’s a debate as well about whether massage parlors should be a law enforcement priority.

Ran, an employee of Restore NYC, a Manhattan nonprofit that combats sex trafficking, says the businesses can treat women badly. She’s seen everything from the confiscation of passports to debt bondage when it comes to compelling women to offer more than therapeutic treatment. She said some women get roped in by responding to vague or deceptive massage-job ads in Chinese-language newspapers, only to find themselves pressured to go further.

“As they begin working, they realize actually a lot of customers are asking them for sex services,” Ran said. And even if they prefer not to engage in happy endings, “they feel like they don’t have another option.”

“A lot of times my clients tell me they work in a massage parlor where massage and sex combine, or it’s just pure sex work, by choice, [but] you’ll always see there’s some reason they can’t find another kind of job.”

But Finckenauer, who spent four years looking into commercial sex work, believes the massage industry is relatively clean compared with brothels and escort services.

At first glance, 93 percent of workers should be considered trafficking victims, according to his Rutgers report, “Researching and Rethinking Sex Trafficking: The Movement of Chinese Women to Asia and the United States for Commercial Sex,” which involved 350 interviews with sex workers, operators, cops and other “key informants.”

That’s because of a common legal definition of trafficking that says a victim is any sex worker who arrives in a foreign country with the help of another person, whether they are charged money or not.

The cops aren’t supposed to take their pants off. So that’s a problem. Most of these transactions occur after the massage starts. - NYPD supervisor, on the difficulty of cracking down on erotic massage parlors

But the premise falls apart upon closer examination.

The researchers found that not a single woman in the group of 149 who left China said she was sold into prostitution or knew of anyone who had been. Just one said she’d been forced, deceived or coerced. And 144 of them had genuine documents and immigrated legally — one was smuggled in and four used forged paperwork. Also, 127 said they were free to do as they pleased.

Two female US attorneys in Brooklyn didn’t uncover much evidence either. They “said that yes, women from mainland China were being exploited in some cases, but not all . . . Asked about coercion, they said it operates in very subtle ways.”

The biggest problem? Debts to “snakeheads” — those who help transport sex workers or provide immigration paperwork. The Rutgers professors found that 68 percent of the women they interviewed owed money after arriving in their new locations.

Ran said women pay $20,000 to $90,000, depending on where they’re from; the debts, which carry interest rates reportedly as high as 12 percent, could potentially compel workers to engage in sex services.

“There is more diversity among the parties involved in prostitution than is commonly supposed, and that to portray them all in the same way as victims is an oversimplification,” the Rutgers researchers wrote. Many of the women were “receptive to or actively sought out the possibility of moving abroad to increase their earnings.”

The goal

Claire is not in debt, and she’s no one’s sex slave. But her life in New York has not worked out the way she’d hoped. She’s not given up on a return to her career in accounting, though for now she sees no viable alternative to massage work.

“Every year I save money so in the future I can buy a house,” she said. “Even if it’s a very small house, I will own it. I won’t need to pay rent or live with another family. And in the future when I have a house and more money, I will be able to go to college.”

She has high hopes for her son, a straight-A student with plans to pursue a medical degree.

“When my son’s a doctor, I will have a good life,” she said. “When I have a regular life, I will be happy.”

This story was written and reported by the Contently Foundation. Visit contently.org.