suddenly two fathers get phone calls in Los Angeles. Hello. His ex-wife maria is about to be arrested. Reporter: We're there as Larry answers the phone, and what he hears, he cannot believe. His... See More
suddenly two fathers get phone calls in Los Angeles. Hello. His ex-wife maria is about to be arrested. Reporter: We're there as Larry answers the phone, and what he hears, he cannot believe. His ex-wife, maria is under arrest. It's surreal, I mean I don't even believe it's happening. You know? Reporter: "20/20" following those leads across Europe. Looking for the fugitive mother. The e-mails. The tips from strangers. And finally that call those fathers had been waiting for. When the first of those calls came, those dads told to rush to the airport. Their destination? Right here in Geneva, Switzerland, one of the richest, most beautiful cities in the world. Maria had been spotted in one of the luxury hotels here. In fact, the tip came from a woman halfway around the world, who heard the story of the mother on the run, the woman e-mailing bob from 6,000 miles away, saying maria had been living here in Geneva with the children and her mother, even putting the children in a school just over the border in France here and one more familiar clue. The woman saying the boys had been introduced to her as Milos and Elie. I want to make sure I got my passport. Reporter: Larry races to the airport in Los Angeles. Geneva via Frankfurt. Reporter: Already there bob, who once fell in love with the same woman -- now both of them so close to catching her, all of this moving at lightning speed. The fathers now rushing to get to Geneva before maria can post bail and flee with the boys again. There are no promises. Just listen to bob on the plane he knows how much is at stake. We're going be really beat up but I got to be strong for jerry and be strong. What a day. What a day. Pictures for his passport. Reporter: His attorney there warning him - she tells us there is still plenty that can go wrong. That the mother might try to get an order from either the French court or the Swiss court to prevent Larry from taking the child home to California. Reporter: Her words prophetic. Because by the time bob's flight arrives a stunning phone call. Reporter: We're there as bob learns maria has made bail. She's trying to get to the boys first. The children will now stay in an orphanage and all hopes of those boys returning to America with their fathers is in jeopardy. Jerry was free two hours ago and now he's not. Reporter: Bob rushes to talk to his French attorney. Who actually saw his son, jerry, before he was whisked away. We listen to her describe what jerry looks like. He's very thin and very long hair. Does he know I'm coming? Yes, I said to him you were coming and he was surprised. Reporter: And bob's attorney says it seems maria is still disguising her boys. She says, even the French police at the school were confused. The policeman policeman said, I take her under my protection. And I said, no it's a boy. So it sucks, it's a major hiccup. I mean, they are not letting anybody see them. There's no way a child is happy in an orphanage, in a system like this, and I'm appalled that I wasn't able to protect my son. They must head to court. As Christmas day approaches, both fathers with perhaps their one Christmas gift, bob and Larry learn they can each have one hour with their sons. It will be the first time they have seen them in 18 months. He's in such horrible shape. He's bone thin, and the cops thought he was a girl. Reporter: We're with Larry as he heads to a French toy store. Armed with a list, but without the language. You know how to say puppets in French? Puppets? Puppets? Yeah! Reporter: Morning arrives, days have passed since they arrived here in France thinking they were bringing their boys home. Instead preparing for that one hour visit. And as we travel with them, Larry reveals to us he's still looking over his shoulder. Good morning. I'm terrified that maria's going to find out that we're here and, you know, she will stop at nothing to get the kids, nothing. Reporter: And bob wonders what his 10-year-old son jerry has been told about him, if he thought his father would ever come. I can't tell you how nervous or anxious I am to see what is up, if he runs in my arms or if he's freaked out and brainwashed. Reporter: Finally, inside those grim walls father's day comes early. We see images of bob and jerry reuniting for the first time. He hugged me and he said, I love you and I missed you. And we just held each other. Reporter: You have been waiting for that moment? Yes, and he was happy. It was just, like, we had never been apart. Reporter: While in another room, Sasha just 4 years old, barely saying a word. He speaks little English anymore, but Larry is determined to break through to his son. I was prepared for the worst, but I could tell he recognized me right away. He knew that I was someone I recognized. Reporter: It goes by quickly. Their hour is up. They return to their hotels, still waiting and still hoping. I'm just feeling overwhelmed right now. He walked me hand in hand and hugged me and said, are you coming back? When do you come back? And when do I get out of here with you? That's what he wants. Reporter: But no one knows what that French judge will decide. As we follow that fugitive mother who was on the run for so long with her boys, we wonder, is she now ready for battle? And what happens when we track her down?
This transcript has been automatically generated and may not be 100% accurate.