Transcript for Mongol Derby: Shocking Twist in Final Moments
America runs on dunkin'. In six days into our epic adventure, hundreds of miles away from the finish line. Out of 30 competitors, 24 remain and they're dropping like proverbial horse flies. Now choices what makes a winner in this, best ride, most strategic alliance or competitive spirit that just will not quit? Every game time decision could be their last. And here again, gloria rivera. Fine. Reporter: Welcome today six of the mother of all equestrian adventures, the mongol derby, devon and laura are locked in a battle over 600 miles of remote wilderness. Both hoping to win the most grueling horse race on earth. One after the other, the mongol derby is taking riders down. Just over half remain. Including 55-year-old rider lynn. Lynn beat back the flu and she found sandra again. Well, we are not filming our good-byes. Ha-ha. No way. Reporter: What started as a tactical alliance has become a deep bond. Sandra is making me emotional. She has been like my little angel. She is taking care of me when i was sick. A miracle. Reporter: Meanwhile five horse stations ahead. It is neck and neck between devon and laura. Both riding at a punishing pace. Because with just two stations left today, only one will be champion. Trying to stay ahead of laura. If you don't believe me. Reporter: Nerves shot they tell us to get lost. You guys need to leave me alone now. We are staying back. We race ahead to the finish line to await the winner of the mongol derby. As of the last horse station it was really neck and neck. Devon a few minutes ahead of laura. Right now we could see any one coming down the stretch. Yeah. There is a rider. A rider! It looks like devon. Reporter: It is devon we see first. Reporter: After seven days of grueling work it seems devon got what she came for. she can't wait to celebrate. Reporter: You stormed in in this competition. And said I am here to win. It will be official. How is he doing? He is still hot. You have plenty of time. Daddy, I just won the mongol derby. They're screaming. But then the vet has crushing news. A two-hour penalty. Reporter: Devon's horse isn't recovering. His heart rate too high. Oh, my god. A devastating two hour penalty its added to her finishing time. Can you belize not fplease not film this? Reporter: Laura arrives a few minutes later. Whoa! Laura. Race headquarters confirms it. Congratulations. Laura is the winner of the mongol derby. Within ear shots of her sobs. She is gracious. Lucky, I got on some horses and managed to get away without any injuries. So I got here somehow. Devon, I am so sorry. It is okay. Congratulations. I really feel for you, that is really unlucky. And unfair. You should have won. I was miles away. I was never going to got here. Just the way things work out. I guess. Okay. Reporter: First place decided we find lynn and sandra at the last horse station ready to finish as a team. I did we would end up best friends or horrible enemies. I think it is going to be best friends. Whoa! Whoa! Reporter: As promised they end the race the way the started it. Hand in hand. If finishing this race seems to mean more to lynn than any one else, well it just might. Remember that secret she didn't want to share with us at the beginning. Well now she does. About three years ago I woke up and got up and fell over and they found I have what is called a hole in your heart. I kind of shut down for six months. Afraid to do anything. And then I just decided that -- you don't have to qui living because you have something wrong with you. A lot of it. Reporter: Devon is here to congratulate lynn with perspective on her heartbreak. The one crappie hour at the finish line did not diminish the week of work I put in to get here. What's done is done. You have got to roll on. You have got to be happy that you finished the mongol derby. Reporter: For "nightline," gloria rivera in the mongolian step.
This transcript has been automatically generated and may not be 100% accurate.