Transcript for The Underground World of Colombia's Child Sex Trafficking Trade
Good evening, and welcome to a special edition of "Nightline." We' this massive undercover sting is not being carried out by police, but Americans, who see themselves as iis abolitionists. David Wright is there. Reporter: Heading out for a vacation, the clear blue water is only part of the allure. We have to play the part of tourists. Reporter: The beauty of the girls attracts plenty of men willing to pay. I had this chick for two days. Yeah. Reporter: These guys are supposedly here for a surprise bachelor party. Kim, the best man in charge of the money. Marcus is bragging he can arrange anything. This girl -- Reporter: Obviously, they're not talking poultry. They're after girls for the big party, and the younger, the better. But they have to speak in code, because what they're looking to do, hire underage prostitutes, is illegal here. Marcus says to be careful. That's not to say he's unwilling to provide if the price is right. Okay. Of the girls that we already have that are sure, how many are min minor? Seriously minor? Six. What are the ages? 15, I got one 13. This is a Colombian operation. Reporter: What's your cover story? We're down here doing a big party from the United States. A bunch of rich guys. I'm the best man from the wedding. My buddy is coming down, and we're going to surprise him with what he loves, child sex. Reporter: He's a former investigator for homeland security. I imagine your work to be sort of the Liam Neeson character in "Taken." The problem, the vast majority of kids, they weren't able to be saved. Reporter: He believes he's rescuing children from slavery. A modern-day abolitionist. Operation under ground rail road has spent months in cooperation with the Colombian officials. Most of the pedophiles are sex tourists, with money and good jobs coming here for something exotic. It's 24 hours before the party. The team knows that tomorrow, anything can happen. Here, okay, and then you're pivoting. Reporter: So they're going over basic hand-to-hand moves. We're not down here on vacation, here to save children. Reporter: S.e.a.l. Team six, this is not. But his group is a group of recruits for the sting operation. Some off the books operatives, but many of them just regular citizens. And this man, a fitness instructor and a door to door salesman. That actress is Laurie holden. Have you ever done something like this? I spent time in Cambodia. I was trained by a Navy S.E.A.L. In "The walking dead." Reporter: Which is easier, this, or killing zombies? We'll see. We'll see tomorrow. Reporter: The group has led operations before in other countries. . We've been in the united States, Haiti, Ecuador. Reporter: How about this group? Are they legit? They're a legit and very professional group. Reporter: They tell us it's not officially sanctioned by the U.S. Government. But they're aware of it. Many of our viewers saying, this is a noble thing. But there's something that feels like, this should be the work of governments. It absolutely should be. If the governments were doing it, we wouldn't be doing it. We got them. Reporter: That's why they travel the world, gathering evidence to give to authoritiauthorities. And a camera crew is following them for a new film, "The abolitionists." And this multimillion-dollar mansion they rented. And this filmmaker wire the place for sound and video for the Colombian officials to get evidence. For weeks, they've been putting the word out to gather girls for the party. Thursday morning, the day of the party, they're getting Se almost showtime, huh? Yes. Reporter: Are you nervous? Yes, but it's good. I think it's going to be weird when we see the girls. It's going to set in, this is who's been sex trafficked. Reporter: This sign making it look like it's a teenager's birthday party. Some of the men splash beer on their faces and clothes. I want them to smell plenty of beer on me. Reporter: Laurie holden's assignment, keep the girls busy in the pool while Tim keeps the traffickers busy. This could be a big challenge, our hearts are going to break the second they walk in. Reporter: But her fame could blow the group's cover. She's going to wear a brown wig. This could work. Reporter: Several different potential traffickers are coming, and their arrivals need to be perfectly coordinated. And they're with the cti, Colombia's version of the FBI. But there's a problem, one of the alleged traffickers are stuck in traffic. They have to get them here, and the girls to a safe place. And every second counts.
This transcript has been automatically generated and may not be 100% accurate.