As he rambles from his Annandale, Va., home northwest to Alaska, the protagonist of Into the Wild spreads happiness and maintains his big-grinned equanimity. Yet there are two reasons to suspect that Christopher McCandless (Emile Hirsch) will not come to a good end. One is that the film is adapted from Jon Krakauer’s nonfiction book, whose conclusion is not a secret; the other is that it was written and directed by Sean Penn, who showed an affinity for men who crash and burn in the three previous features he’s directed. Yet this tale is something of a switch for Penn, and it has a sense of acceptance and possibility that makes it his most satisfying work.

True to Chris’ adolescent romanticism, Into the Wild begins with a nature-struck verse from Lord Byron and the first burblings of an earnest art-folk score (by Michael Brook, Eddie Vedder, and Kaki King). Hopping between the past and the more recent past, the opening scenes sketch the young man’s pre-nomadic life while depicting his arrival at an abandoned bus in the Alaskan wilderness, where he’ll settle down after some two years on the road. The dutiful son of bickering, overpowering parents (William Hurt and Marcia Gay Harden), Chris has done his time at Emory University and gotten a degree. The rest of Dad’s plans for him, however, are to be shelved. Without even informing his beloved little sister, Carine (Jena Malone), Chris gives away his remaining money and hitchhikes west. He is now, he declares, Alexander Supertramp. (Cue Canned Heat’s “Going Up the Country,” one of the most overused soundtrack ditties of the last 40 years.)

Chris spends a long season alone in Alaska, with voiceover of his letters, passages from Tolstoy, and hand-carved diary entries substituting for dialogue. Into the Wild lacks the imaginary friendships with local fauna that characterize such alone-in-the-wilderness chronicles as Grizzly Man and Never Cry Wolf, but the movie doesn’t attempt to conjure the experience of a solitary existence. Instead, it flashes back regularly to Chris’ pre-Alaska encounters, which largely revolve around surrogate families. Peripatetic hippie couple Jan and Rainey (Catherine Keener and Brian Dierker) take in the drifter. An earthy South Dakota grain dealer (Vince Vaughn) briefly becomes the big brother Chris might have had. And a lonely California desert-dweller (Hal Holbrook) auditions for the role of honorary grandpa. Everyone the wanderer meets is improved by the contact, a notion that’s even more romantic than Chris’ conception of the wilderness.

“You’re not Jesus, are you?” Rainey asks Chris, and Penn is almost willing to make that claim. If the young traveler is only a minor-league savior, he’s accorded a glow worthy of Renaissance religious canvases. What’s least believable is that Chris has no appetites save for friendship and adventure. He makes no major forays into drugs and booze, and he declines to sleep with a teenage girl (underage temptress Kristen Stewart) who couldn’t advertise her availability any more openly. (In place of hot sex, he entices her to join him for a tepid duet of John Prine’s “Angel From Montgomery.”) Into the Wild refuses to acknowledge that healthy 22-year-old guys do not live by white-water kayaking alone.

Remarkably, however, Hirsch makes the sainted Chris believable and sympathetic. The baby-faced actor, who conveyed little more than callowness in such films as The Dangerous Lives of Altar Boys and The Emperor’s Club, benefits from a few added years, scraggly facial hair, and Penn’s direction. Hirsch leads a cast that’s both grounded and light on its feet, a description that can be applied to the film itself. Penn’s empathy with Chris is evident, yet the director concentrates not on the kid’s fate but on his expansive character. For the first time, Penn has made a movie that sees alpha-male obsessiveness as a path to liberation rather than doom. Reasonable people may debate the meaning of Christopher McCandless’ saga, but Into the Wild makes a credible and even joyous case that the young explorer gained more than he lost.