(Sony Pictures Classics/Maryse Alberti) (Sony Pictures Classics/Maryse Alberti)

Documentarian Alex Gibney is considering making his next film about something nice — puppies, perhaps. One problem, he says: “All the puppies would die.”

Gibney, director of “The Armstrong Lie,” opening Friday, can’t seem to find himself a happy subject. His Oscar-winning 2007 doc “Taxi to the Dark Side” was about an Afghan cab driver who was tortured and killed by U.S. forces. 2005’s “Enron: The Smartest Guys in the Room” chronicled a U.S. company’s crash and burn. 2012’s “Mea Maxima Culpa: Silence in the House of God” covered child sexual abuse in the Catholic Church. And earlier this year, Gibney released the phenomenal “We Steal Secrets: The Story of WikiLeaks.”

Gibney thought he’d found his nice story when he decided to capture Lance Armstrong’s triumphant return to cycling as he set out in 2008 to win the Tour de France for an eighth time. It would be one of those inspiring sports films: Not only was Armstrong re-emerging from retirement, he was doing so as a cancer survivor. (Armstrong finished third in the 2009 Tour.)

And then came Oprah.

In January, Armstrong admitted in an exclusive interview with Oprah Winfrey that he used performance-enhancing drugs, both in the comeback and in his previous seven consecutive Tour de France wins.

Gibney had a whole new film on his hands: He had hours upon hours of footage of Armstrong denying his drug use while God only knows what was coursing through his veins.

“I get lied to all the time,” Gibney says. “It’s part of my job description.” This time, though, he got sucked in.

“Part of the problem was the beautiful lie was far more compelling — you know, hope rides again! I know a lot of cancer survivors and cancer victims who obtained a lot of inspiration from Lance Armstrong and his story, and that was real,” the director says. “It was beautiful, the idea that by the dint of your will you can make a difference.”

Aware the story had changed, Gibney arranged for one final inter- view with Armstrong. In that foot- age, Armstrong appears contrite as he admits his misdeeds.

“I think there are some moments [in the final interview] where he’s telling the truth,” Gibney says. “But having been lied to so many times, it’s really hard to trust him. But I’m pretty sure it’s the truth.”