IN THIS SEASON of discontent, it seems like every major documentary is an exposé. Credit an overabundance of material, thanks to the folks at Big Oil and on Wall Street, and the success of films such as The Cove and Gasland. It’s surprising, then, to discover that this summer’s most urgent documentary contains no news whatsoever. Marshall Curry’s If a Tree Falls: A Story of the Earth Liberation Front, which premiered at the Sundance Film Festival and goes into select release this month, revives a narrative that stretches back into the late nineties. In 2005, FBI agents stormed the Manhattan office of Womens­Law.org, a nonprofit run by Curry’s wife, and arrested a soft-spoken employee, then-31-year-old Daniel McGowan. In a previous life, McGowan was a member of the Earth Liberation Front (ELF), the radical eco-activist—or, depending on your point of view, terrorist—group best known for a series of arsons in Oregon and in Vail, Colorado. Intrigued, Curry rolled the camera, followed McGowan in the days leading up to his trial, and set about finding out how a polite former track star from Brooklyn ended up torching an Oregon tree farm.

The result is thrilling. Using archival footage, some of which has never been seen before, Curry re-creates the story of the ELF’s founding in Eugene, Oregon. The group’s leader is an enigmatic young activist named Jake Ferguson with a taste for heroin; our hero, McGowan, comes off as a quiet, principled man in way over his head. What makes If a Tree Falls so gripping is Curry’s reconstruction of some of the ELF’s most famous capers. “I’m standing there,” says McGowan in his matter-of-fact alto, “I’m drenched in gasoline; we’re about to burn 13 huge SUVs.” The vibe alternates between an early Clash concert and a dark, dark twist on Ocean’s 11. Then things begin to fall apart for the young fire starters. The ELF disbands, in part because some members want to escalate the violence, and McGowan starts a new life in New York City, ultimately realizing that his passion for the cause can’t compete with his love for family. But the bottom drops out when a paranoid, smack-addled Ferguson turns rat and the ax of the law falls. While everyone else is making documentaries about the ills of the world—and God bless ’em for that—Curry does that old, simple, beautiful thing. He finds an incredible story and simply gets out of the way.