It was a beautiful -- and a liar was one of the most celebrated sports figures in the world's. Lance Armstrong reached the pinnacle of athletic achievement winning the Tour de France year after year... See More
It was a beautiful -- and a liar was one of the most celebrated sports figures in the world's. Lance Armstrong reached the pinnacle of athletic achievement winning the Tour de France year after year even after a battle with cancer. But when the -- unraveled so -- life as he knew it's now the former American hero explains how it took over and spiraled out of control. Here's ABC's Neal Karlinsky. You're inside one of the most challenging sporting events on earth with access no outsider -- ever had to Lance Armstrong. At the time still considered a seven time winner of the board of problems. But what Academy Award winning filmmaker Alex -- he and his studio didn't know despite a year in sight Lance Armstrong's normally private bubble. Was that it was all a lie and they too were being played a lot of people wonder why would Lance talked. I think that particularly at the beginning there was a sense of invulnerability. The sense that. Nobody is ever gonna discover my -- so come along for the ride the ride ended up traversing Armstrong's epic collapse leaving kidney with moments he never imagined capturing. I certainly was very confident that I would never be -- A shell shocked Lance Armstrong matter of factly explaining how it all began. His decision to cheat by using performance enhancing drugs. It was a group of us primarily live in and Italy and we just said we either have to play ball here -- -- home. We just thought that she had it was just happens to have an order. The latter. But but but but but. -- -- -- Maybe -- that approach the decision differently today but at the time I didn't lose sleep over. It is a slice of the new film the Armstrong -- Which dissect what's been called the biggest fraud in all of sport and explores Howell -- became so all consuming Armstrong would do almost anything to protect it. I have never -- to have never taken performs in his -- my best defenses I've never tested positive how do you rate Lance is a liar. And even more than that as a copper up artist he is. -- tippy top he's one of the very best. The Armstrong -- is a film -- -- never intended to me. He was directing a feel good story about Armstrong's 2009 comeback. They finished it complete with narration by Matt Damon but before they could release it. Tonight on Nightline did Lance -- Nightline ran a tell all former teammate Floyd Landis who dropped out bomb. Did you see -- Armstrong receiving transfusions -- more than -- -- multiple times. Look at some point people have to tell their kids and Santa Claus isn't real. I hate -- do it -- It's just not real. They put the film on the shelf and re shot a completely different story it was Lance is abuse of power. That is really the more reprehensible -- part of it was the way he went after people particle was a way he actually wrapped himself in the -- Of the cancer survivor he first confessed to Oprah. Did you ever take banned substances to enhance your cycling performance yes. Did you ever blood dope or use blood transfusions to enhance -- -- performance yes. And all seven of your Tour de France victories did you ever take banned substances or plateau that's. Armstrong talked to give me. Hours later live in -- line. I didn't live. A lot of allies -- other one being -- and you know it's different I guess it's time for some reason when he went on Oprah. It just dropped like a -- We think barometer. Part of the problem was Lance is -- used economically. And I think you saw that discomfort on Oprah. And I think he himself realizes what complete disaster was. Photos from behind the scenes with -- -- show a fallen champion who has lost some of his swagger still Armstrong trusted give me from their earlier time together and for the first time. Agreed to explain some of his secrets including how he got away with using the blood booster EPO. The half life of EPO is. Four hours. So. You can -- it out from there and figure out. And when you're in trouble. -- he passed every test because he does not take you deal. Yes he will do my defense was that -- passed server control you give them and that's true the samples that were given work -- -- is great tragedy was that he couldn't see the difference he couldn't see the borderline. Between where the sport ended and we're real life began. Much of Armstrong's life today centers around trying to keep what he has left. Three major lawsuits including a fraud case brought by the US Department of Justice they seek to strip him of more than 100. Million dollars. You talk to anymore the last time like communicated with Lance was when I let him know that for. The film was going to be called the Armstrong -- Product or not so well if his reputation stands a chance that any kind of redemption. It will likely take awhile. People don't like that truth as much as they liked the beautiful -- And that's a hard thing from -- to accept because he found much more affection. Beautiful life and the ugly truth. -- Neal Karlinsky for Nightline in Los Angeles.
This transcript has been automatically generated and may not be 100% accurate.