 -- ABC's John Kapetaneas is a producer for "Nightline." This is his account of a report he produced with ABC's Bob Woodruff and Karson Yiu.

"How can I help you?" an undercover informant asked in broken English.

"Yeah," the man replied, asking without hesitation "where I can find sex with young girls?”

His intonation was sharp and his request sounded crystal clear. We were looking at a video of a grown man, possibly American, purportedly looking for young girls for sex.

Within the first hour of meeting former pastor Don Brewster, he pulled out his iPad to show us hidden camera video of what looked and sounded like western men -- 3 Americans and 1 Australian to our ears -- asking strangers on the streets of Cambodia where to find children for sex. The video was filmed covertly by members of his staff over the last two years in the neighborhood of Svay Pak, just 10 miles outside the capital of Phnom Penh, where Don and his wife Bridget now live.

Our team sat mouths ajar. You could hear a pin drop in the room as the video played:

Undercover: How old do you want?

Man: I don’t know, 10?

Undercover: 10? 10 years old?

Don and Bridget are the founders of Agape International Missions, or AIM, a Christian missionary group on the front lines of the war against child sex trafficking in Cambodia. They are both in their 60s and have 12 grandchildren back home, in Northern California and Upstate New York.

In 2005, following a visit to Cambodia where Don said he was propositioned by a pimp offering him young girls on the street, Don and Bridget decided to pick up their lives in the U.S. and moved there full-time. They have made it their life’s work to stop men like those in the undercover video, from abusing children.

The Brewsters welcome us to Phnom Penh with a smile -- and a warning: Not everything we were about to see would be easy to handle.

At the center of their operation is a restoration home in Phnom Penh for young girls rescued from the sex trade. There are more than 50 children living there the day we arrive, many of whom are less than 10 years old. Walking through the halls, where the kids run around and play, we’re struck by a sense of normalcy. Hand prints are painted on white walls. Teddy bears line each of their bunk beds. Little girls, trying their best pop-star impressions, sing along to Cambodian tunes on their own karaoke machine. Staff members throw high fives to passing kids, while Don and Bridget share hugs with every girl that wants one.

It’s hard for us to imagine anything other than these children being children. But lurking in the ether is the shared horror they all once faced.

Human trafficking was once endemic in Cambodia. Over the decades that followed the rise and fall of the Khmer Rouge in the late 1970’s, the country had developed a reputation as a hotbed for trafficking and child prostitution. The U.S. State Department frequently listed Cambodia as either a Tier 3 or Tier 2 “Watch List” country, among the worst ratings a nation could receive for efforts to prevent human trafficking. Anecdotal stories are even more stunning: tales of pimps and profiteers openly selling women and children on the streets are commonplace.

William Heidt, the current United States Ambassador to Cambodia, lived and worked as an officer for the State Department in Cambodia in the late 1990s. “That was the heydey of Cambodia as a sex tourism destination,” he tells Nightline in a recent interview. At the time, he said he would often take delegations visiting the country to the brothel district to witness the issue firsthand.

"We would see these tawdry brothels, some with what appeared to be children standing in front of them," he said. "It was very sad."

Many reports now indicate that that reputation has been drastically waning. According to surveys of sex workers on the level of prostitution in the country, the number of child prostitutes has reportedly been dropping for years. Survey estimates once reported nearly 35 percent of Cambodian prostitutes, numbering around 5,000 total, were minors. Recent estimates pegged that number closer to 1 to 2 percent, which would be a major statistical improvement, but is still a figure considered far too high by officials.

But Don Brewster believes the falling estimates are misleading. In a Washington Post op-ed he penned in 2015, Don claimed the problem of child prostitution in Cambodia is “far from over” and in many ways continues to thrive. The major issue, he said, was the migration of the sex industry underground, following crackdown efforts by the government, law enforcement and international agencies. No longer are women and children openly sold on the streets of Phnom Penh and Svay Pak. Rather, they are allegedly sold out of more covert locations, like a massage parlor. Or a beer garden. Or one of the many karaoke bars lining certain streets on any city or country road.

If you were to drive through some of these streets, as our team did during a recent trip to Phnom Penh, you might be hard-pressed to conclude there was no issue. Don takes us down one disheveled dirt road lined with bars and neon sign after neon sign reading “KTV.” Karaoke bars, Don told us, are just one of the fronts for brothels in Cambodia. In his estimation, 80 percent of them don’t just sell karaoke, they also sell sex.

At one point, a young woman approached our car. “Boom, boom?” she freely offered. Only $15, she said, then dropped the price to $10 when we politely refused. We drove away.

Don told us many of these places will parade a line of young women for you to choose from. Undercover video he would later show us of a suspected brothel confirmed that statement: rows of young women, scantily clad, lined up on stadium style bleachers. Allegedly waiting for a buyer.

But Don explains to us that even these cases are not the most pressing issue at hand. Out of sight, out of mind, in back rooms and dark concealed locations, he said, underage girls at certain places are still being sold for sex to pedophiles. Locked away where they can’t be seen or heard. Don said they are saved for special customers that the traffickers trust.

Agape’s compound in Svay Pak is a harsh reminder of that practice. They operate a number of programs in at least half a dozen buildings in the community: schools, medical clinics, employment centers, daycares and homes for abandoned children. Every one of the buildings, where kids now run and play freely, was once a brothel where children as young as 8 years old were trapped, abused and sold for sex.

In the far back of one building, Don shows us a cement lined room, colored pink, with the number “9” emblazoned on the outside. This was just one of any rooms like it where young girls were held. A wooden cot on the ground, surrounded by cement walls, with no place to escape. Conjuring up images of a prison cell in some destitute penitentiary, rather than a room where any child should ever be forced to be. It’s hard not to be angry in a place like this.

Sokha's Story

For five years, that was Sokha Chan’s entire world.

The first time we met Sokha, we instantly were captivated by her spirit. Twenty-two years old, she lives and works as a nail technician in the suburbs of Sacramento. In her spare time, she bakes cakes at a bakery, goes to church and plays with her lapdog, Chica.

In almost every way, Sokha is your typical Bay Area California girl. But the stories she shares come from another world, a nightmare reality we’d like to believe didn’t exist. Underneath her smile and joyous personality, we knew this was someone with a past so horrifying, so inexplicably filled with pain and injustice that it defied all understanding of what a childhood should be.

Sokha told us that when she was just a 7-year-old girl, she was sold in a virgin sale to an American man named Michael Joseph Pepe, a former Marine living as an expat in Phnom Penh. She told us in agonizing detail the horrific atrocities committed against her by this man. The abuse was so bad it landed her in the hospital. And her nightmare just began there.

Sokha told us she was sold again to a trafficker, who shipped her to brothels all over Cambodia and Vietnam for years. Even in her agony, Sokha showed the fortitude that would come to define her life. In one harrowing example, she told us, two other children in a brothel where she was being held were sold to a buyer for the evening. The girls were terrified, so Sokha defiantly took their place, despite knowing that she would take a beating for slighting the brothel owners. Every night she was locked in a dark room.

She recounted these stories with tears in her eyes. We were misty-eyed just listening. She told us that she would cry every single day in the brothels. She was scared every moment.

And then one day, suddenly, everything changed.

Sokha told us a man came in one day, only to play pool and drink. When he left, uniformed officers stormed the brothel, arrested the traffickers and rescued her. It was a raid, spearheaded by Cambodian National Police and NGOs working in Cambodia, targeting the brothel she was in. For the first time in years, at the tender age of 12, Sokha was free.

When we visited Don and Bridget at their home in Svay Pak, Sokha had been staying with them, sleeping on their couch. It felt almost like a daughter home from college visiting her parents. It’s a stunning reversal from the little girl they met 10 years earlier, who they brought into their restoration program after a childhood spent in the sex trade.

After testifying in 2008 in U.S. Federal Court against Pepe, who was later convicted and sentenced to 210 years in prison, Sokha was able to obtain a visa to move to the United States. This was her first trip back to Cambodia since the trial. She was seeing her two little sisters for the first time, both of whom were brought into the AIM program at her request.

AIM told us that every day little girls are sold into the sex trade and they needed to do everything they could to stop it.

Christian Missionaries by Day, SWAT Team By Night

AIM felt the efforts to combat child sex trafficking were not enough. While there were police raids and operations by other NGOs to combat sex trafficking, they saw the existing efforts as insufficient.

One example Don often cites is a pimp who approached him one day while walking through Svay Pak, where he and Bridget now live. That pimp had recently been arrested in another brothel raid where underage girls were being sold. Yet, there he was, on the streets again, allegedly offering Don young girls to purchase. He was brought into a room where the pimp had over 60 young girls to choose from.

As a response, in late 2014, with the help of some generous donors, AIM created its own SWAT team to investigate and execute rescue operations in illegal brothels suspected of selling underage girls.

We meet Eric Meldrum, a Scottish-born former police officer in the U.K., who was brought in by AIM to lead their SWAT program in Cambodia.

Based on their mission, to put an end to child sex trafficking in Cambodia, AIM obtained legal authority through multiple departments of the Cambodian government to establish a team of brothel-raiders. They work in tandem with the Cambodian National Police Anti-Human Trafficking Unit, jointly executing missions on suspected perpetrators and establishments.

“Kill the chicken to scare the monkey” is written on a dry erase board on the wall in the AIM SWAT office. It’s a Chinese idiom: make an example of one to scare the others.

When AIM offered Nightline the opportunity to go along with them on their next raid, my team did not know exactly what to expect. Eric and his investigators had spent weeks conducting their intelligence operations. They lined up three possible targets: the most urgent, they believed, had six to eight trafficking victims. At least one was a minor, he said, possibly more. A massage parlor, just several miles away from their office. The perpetrator? A woman they say is a notorious sex trafficker known as “the mama-san.”

Their intel had come from a 21-year-old woman who recently escaped said massage parlor. The mama-san, she told AIM, would beat and abuse her and sell her for sex every night for just $3. She said there were minors inside.

AIM investigators went covertly inside the brothel to gather intel. They’d go in waves, sending back updates via phone calls to the team. The mission was expedited based on that info. AIM and their police counterparts were mobilized. Bullet-proof vests and body cams were thrown on. A battering ram was loaded in the trunk of Eric’s truck. Guns were holstered for everyone except Eric. Like most U.K. police officers, Eric doesn’t carry one.

At police headquarters, we were quickly shuttled inside to a makeshift command center. They didn’t want a team of westerners holding cameras hanging around outside for too long, they said, somebody could get suspicious about what was going on. Inside, police generals talked back and forth in Khmer, with each other and with their men inside the brothel. At one point, everyone inside jumped to their feet as the local prosecutor walked in. He gave the final authority for the raid to happen. We just needed the greenlight from the undercovers inside.

Minutes began to turn into hours. We stood by, waiting for the final go-signal when the call came in: the girls had been sold privately for the night. They wouldn’t be back. The mission was called off.

That night, we couldn’t help but think about those girls that we couldn’t rescue. Sokha had once told us that every night she was in the brothel, she cried, terrified of the abuse, scared of the men she would be called out to see. She believed she would die in there.

Early the next morning, I woke up to a text from Don: the mission was back on. We had to get to the SWAT team office.

Moments before we stormed in to the alleged brothel, Eric seemed tense. He drummed on his gear shift, like an internal metronome ready to crank into gear at a seconds notice. He was always a bit nervous during these raids, he said, because you never knew what was going to happen despite having done it many times.

Fortunately, the raid goes off without a hitch. The mama-san puts up no resistance, and easily gives herself up to authorities.. From a distance, we hear the click of heels coming down the steps. Two young girls emerge, both believed to be underage, just teenagers. To us, they look even younger. One wearing stripes, the other in a Nike shirt, they could be any teen you see at any suburban strip-mall on any given day. Instead they were here. But not for long.

The SWAT team isolated them across the room. They looked distressed, their faces in their hands. They may not have known what would happen next.

Just outside, a white van pulled up to the police tape. One by one, a team of young women crossed the tape, high fiving Don Brewster on the way in. They were all social workers from AIM, whose sole job was to help transition these girls from this life of deprivation and exploitation, to freedom.

Facing the Past

Just three days after the girls were rescued, they were brought to AIM’s restoration home, where children who were victims of trauma and sexual exploitation were brought to heal. The team lined them up with teddy bears and place a tiara on each of them. It’s a long running AIM ritual, Don told us.

Every rescued girl is crowned a princess. Just as Sokha was 10 years ago, the first day she was rescued, the first day she met Don and Bridget, the first day she said she was truly free.

Sokha chose to come visit her homeland once again for a number of reasons: she wanted to see her family, particularly her sisters and she wanted to once again see the people who she said saved her life, especially Don and Bridget.

But she also wanted to do something we never expected. She wanted, for the first time, to confront the person who both she and AIM say sold her to an American pedophile 15 years ago: her own mother.

As our team sat with Sokha, Don and Bridget, idling in our vehicle just yards away from Sokha’s childhood home, you could sense a quiet confidence from this young woman, who in a short lifetime had experienced so much pain. Her mother was the first person to greet her when she got to Cambodia, they even shared an emotional hug. She had gone to visit her several times during her trip back before this day. But never before had Sokha questioned her mother directly about her past. About all the pain she experienced.

As we walked-up, a woman sitting on a small wooden stoop greeted us with a wai, a prayer like greeting. We say hello in Khmer, Suo-Stei, as she invited us inside. Pictures of Sokha’s life in California lined the wall of their home. Sokha told us her father has been putting them up throughout the years. There were family pictures of Sokha’s parents with her two younger sisters, but she isn’t in any of them. In fact, we notice, there are no pictures from her childhood at all.

That childhood, in many ways, ended when she was just a 7-year-old girl.

The confrontation is incredibly wrenching, as you can see watching our story. She denies selling Sokha for the purposes of sex, saying rather that she sold her to be a waitress at a cafe. And before that, a maid to Michael Pepe. Our viewers can form their own opinions about what Sokha’s mother did or did not do. But what struck us so intently was Sokha’s strength of character. She seems to refuse to let that heart shattering 5 years define her. Nothing is more evident than when, without provocation, we notice she bows her head to her mother. A gesture of forgiveness, she tells us. From a daughter, to her mother.

The Path Forward

Despite some of the tragic stories we witnessed firsthand during our reporting, we also consistently witnessed an overwhelming feeling of belief. Belief that there is hope for the future in Cambodia. Belief that came in the form of unshakeable, inspiring young women like Sokha. Belief that came in the form of a tireless couple and their team of faith-based warriors, working to do good in the face of evil.

U.S. officials say Cambodian authorities have made great strides in combating the illicit sex trade. The Cambodian Police unit who spearheaded the raid we accompanied appeared to be ardent professionals, dedicated to the mission of eradicating sex trafficking in their country.

“They need a consistent sustained principled law enforcement (effort) in the future,” Ambassador Heidt told us. “I don’t know if there's’ a magic bullet, but they need to keep out drugs, and their law enforcement effort needs to reach down to those underground places: KTV’s, beer gardens … and they'll be able to put this issue fully in their rear view mirror in the next 10 years if they do that.”

Meanwhile, the fight against child sex trafficking continues.

Because every day, NGOs like AIM are changing the lives of women and girls all over the small country of 15 million.

Don and Bridget Brewster have no thoughts of leaving Cambodia, not at least anytime soon. Their grandchildren ask when they will come home and that is always a consideration.

“Did you ever think in a million years that you’d retire in a place like this,” we ask. “Heck no!” they respond without hesitation.

But still, they could not imagine leaving. Not yet. Not while young girls like Sokha are still out there. Not while daughters are still for sale.

As parents, it may be hard to imagine. That a mother could ever sell a daughter. That a sold daughter could ever forgive.

But when you meet someone like Sokha Chan, you see reserves that defy years of the cruelty, a small lifetime of neglect.

She told us she now wants to fight against child sex trafficking. And she wants to help other girls just like her.

Perhaps it took the kindness of these complete strangers, moving from the other side of the world. It took years and years of care, to begin reversing the horrors of her youth. But the years of darkness, fear and despair are now filled with hope, forgiveness and love. Love for others. Love for her faith. And most of all, love for herself.

In our last moments with Sokha, we asked her if she ever had any love growing up. “No,” she told us, “I feel like I am the ugly one in the world.”

And now?

Without missing a beat. “Now, I am a princess,” she said. A smile and a laugh follow. No one could convince her otherwise.

ABC News' Gamay Palacios contributed to this report