It's just 8:30 in the morning when I spot the nudists. Outside the catering truck, I'm interviewing Into the Wild executive producer Frank Hildebrand when a white GMC Yukon rolls up to the parking area next to us and a leathery, hirsute man wearing a light-blue, knee-length bathrobe gets out of the passenger-side door. The wind blows, his robe flies up, and—whoa!

"When Chris was camped out here," explains Hildebrand, "he was just up the road from Oh-My-God Hot Springs." We're standing outside California's Anza-Borrego Desert State Park, a barren no-man's-land of ocotillo-lined red-rock mountains and scorched bajada stretching to the desolate Salton Sea. "These people are extras in the scene we're filming," he says, before adding the obvious: "But they're real nudists."

Chris is Christopher McCandless, the 22-year-old from Annandale, Virginia, who graduated from Atlanta's Emory University in 1990, donated the remaining $24,000 in his college fund to Oxfam America, cut ties to his parents, and took off on a quest to escape his privileged upbringing. For two years, he wandered North America alone as "Alexander Supertramp," abandoning his car in the Arizona desert and then riding trains and hitchhiking from California to South Dakota to Oregon to Utah to Washington to Baja and points in between. Then, in the spring of 1992, he walked into the Alaskan wilderness for his final adventure. Four months later, trapped by a swollen river that had cut him off from civilization, he starved to death in an abandoned Fairbanks city bus.

After detailing McCandless's tragic odyssey in the January 1993 issue of Outside, Jon Krakauer expanded the story into 1996's Into the Wild. The book spent 103 weeks on the New York Times bestseller list and is now taught in English classes all over the country, a sort of nonfiction Catcher in the Rye that takes on the same issues of family dysfunction, misguided youth, wanderlust, and the pitfalls of an unexamined life. At its heart, it is also a classic adventure story. Over the years, hundreds of readers have been moved to write to the McCandless family; others have dropped everything and made pilgrimages to visit the places Chris traveled, including the remote Alaskan bus where he died.

Filmmakers have been equally gripped by the book, none more than director and Academy Award-winning actor Sean Penn, who's spent the past ten years trying to make a movie out of Into the Wild. "The story just touched a chord," says Penn, describing his discovery of the book back in 1996. "One part was trust issues in a family and society at large, but the greater aspect was this wanderlust that everybody shares in."

In the fall of 2005, Krakauer and the McCandless family finally signed off on Penn's plans for a movie (the film hits theaters September 21), in part because of his pledge to stick so closely to the true story. The next spring, Penn hit the road with an all-star cast and crew—including actors Emile Hirsch, Vince Vaughn, Catherine Keener, William Hurt, and Marcia Gay Harden and Brokeback Mountain executive producer William Pohlad. For six months, the 150-member team has crisscrossed the country, shooting at 36 locations McCandless had visited. Penn was fanatical about the details. Everything has been exhaustively accurate, from the facial hair of the star (22-year-old Hirsch plays McCandless) to the make and model of the Alaskan bus to, yes, the presence of real nudists on the set.

Today is the 78th day of shooting. Penn, wearing jeans, a light-green T-shirt, work boots, and a straw cowboy hat, is blocking out a few movements with Hirsch for the next scene. McCandless will get dropped off at his wilderness campsite by Ronald Franz (played by veteran character actor Hal Holbrook), an 80-year-old Army veteran from nearby Salton City who befriended McCandless and eventually offered to adopt him.

"Let's use the 'shut the fuck up' rule," says Penn, to no one in particular. The crew goes silent and we watch as the scene unfolds.

Action!

Franz's Ford Bronco approaches the spare campsite and the two get out. McCandless sits on the hood of the truck—just as Penn had blocked it out—and explains to Franz his philosophy on getting a job. "I think careers are a 20th-century invention," he says. He's wearing a red T-shirt, short blue running shorts, and New Balance running shoes. "I've got a college education. I'm not destitute. I'm living like this by choice."

"In the dirt?" asks Franz, looking at the small green tarp McCandless has tied between two creosote bushes for shelter. "Where's your family?"

"I don't have a family anymore," says McCandless.

In fact, McCandless's parents, Walt and Billie, and his younger sister, Carine, are all here on the set today. Carine is standing a few feet away and watches the scene play out for three more takes. This convergence of reality and fiction is both startling and heartbreaking. But then, how Penn, this crew, and the surviving family all banded together to tell McCandless's story is an unbelievable tale itself. Here, from the beginning, they reveal it in their own words.