Now we want to turn to David Muir and an ABC news exclusive, thy storof two beautiful little boys who suddenly disappear when their mom takes they will on a vacation and never returns leaving two... See More
Now we want to turn to David Muir and an ABC news exclusive, thy storof two beautiful little boys who suddenly disappear when their mom takes they will on a vacation and never returns leaving two ex-husbands, two fathers so heartbroken and what follows is an extraordinary international hunt to find the mother on the run and as I said, David Muir has been on the trail on this story for months. It's remarkable. This is really something. We were all talking about how heart pounding this is. In fact, 18 months go by, two dads, two ex-husbands have no idea where their beloved boys are. When suddenly a piece of evidence, a video breaking the case wide open. This morning you're about to see those stunning images, how she'd been disguising the boys. The Hollywood hills where scripts and screenplays are filled with fictional family drama but right here in los Angeles two different fathers finding themselves in the middle of a real-life stunner. For 18 months both of them asking where are their sons. Will the mother, the ex-wife who took them ever be found? Jerry is everything to me. Reporter: Can you hear his voice still. It's strange. I feel like I'm with him all the time except he's not here. Reporter: The youngest, Sasha who loved his drum set. And jerry, who loved to be tickled at night. Both boys taken overseas on vacation by their mother but on previous trips maria Pfeiffer had always come back. Not this time, it landed in los Angeles and they are not on it. She went dark on her phones and went dark on e-mails. Reporter: Somewhere in Europe a fugitive mother. Back here two different dads working to track her down and we begin to follow the leads from Geneva to Paris, the e-mails, the clues she's leaving behind. One of the strongest tips leading to Paris. Agent Casey. Welcome to Paris. Reporter: One of the dads showing up unannounced armed with evidence pleading for help from the FBI. Had you ever seen a case like this before? A dad who shows up at the Paris embassy? No. Reporter: We hunt down the clues, an apartment building where a Marie that Pfeifer lives. And then the biggest clue of all. Look at this. The stunning video. The boys playing at a resort in Slovakia caught on tape. More than a year after they disappeared. You can see their mother there in the Orange shorts and what's most startling to their fathers is how they look. Both of the boys with long hair, the youngest in pigtails. That's Sasha to the left? That's Sasha in the pigtails disguised as a girl in the yellow shirt. Jerry is coming around right here, right? Jerry is in -- with the long hair. Reporter: How strange is it to see your son disguised as a little girl halfway around the world. It's surreal. Reporter: The fathers showed the video to us and to the FBI. The international dragnet tightening. I'm waiting right now for the word from law enforcement to get on a plane and go to Europe. Reporter: You hope that call comes. I dream of and pray that call comes. Reporter: And what happens when we track down that mother? We do track down that mother and tonight you'll hear from herself how she explains this whole ordeal. We're with the fathers every step of the way this evening the very moment they get the call from Europe. The big question, of course, can they get to the boys first? The moment they discover them. That's tonight on "20/20." Every year a thousand children are abducted by parents in this country and taken overseas to foreign countries. To be there when all this is happening is just incredible. Yeah, this is really something and you got to think about the children who are caught right in the middle of this thing? And video, that pivotal video and how they got it is unbelievable. With help from perfect strangers all over the world on Facebook.
This transcript has been automatically generated and may not be 100% accurate.