Sean Penn’s uncompromising, unsettling, uniquely satisfying film version of Jon Krakauer’s best-seller, “Into the Wild,” tells the true story of Christopher McCandless, a young man from a wealthy Eastern family who made an ill-fated trek into the Alaskan wilderness in 1992.

The movie is very much its own thing: A strange study of that impulse that, for generations, has made so many Americans want to leave civilization and push west — and a melancholy acknowledgement that, for most of these people, the journey offers little fulfillment.

It’s a celebration of the survivalist instinct and, at the same time, a warning of the dangers of idealizing nature — a scary, cautionary tale of just how perilous and unfeeling the world can be outside the network of human society.

With a little-known star (Emile Hirsch), a nearly three-hour running time and a downer story line, the film seems guaranteed for box-office oblivion, but it certainly strikes me as Hollywood’s most unforgettable and haunting film thus far in 2007.

The script, which Penn also wrote, picks up its hero’s story two years after he has disappeared from his family, changed his name, given away his money, hitchhiked around the country and, unprepared, is about to enter the Alaskan wilderness north of Mount McKinley.

Into the wild - Christopher McCandless

From here, we follow his survival story as he takes up a Robinson Crusoe-like existence in an abandoned bus he stumbles upon for shelter. Flashbacks trace the story of his dysfunctional family life — most of it narrated by his younger sister (Jena Malone).

Flashbacks also trace the backpacking journey he began after college, during which he briefly bonded with a farmer in South Dakota (Vince Vaughn), a pair of snow-birding ex-hippies (Catherine Keener and Brian Dierker) and, most poignantly, an Army retiree who becomes his surrogate father (Hal Holbrook).

As the story proceeds on three levels, the magic of the movie is that it doesn’t try to understand or judge McCandless: He remains an enigma who can be seen as a pretentious rich kid on a lark, as a noble existential adventurer, or as a tormented, and apparently asexual, victim of a lethal family life. In a sense, he’s all three. But it’s the mystery of his selfishness that gives him real power as a character. His character shows just how impossible it is to neatly define or know another human being.

As in Krakauer’s book, the character’s odyssey toward doom becomes a symbol of man’s lonely fate in the cosmos and the contradictory fact that everything we do affects the people around us. Indeed, the film’s most gut-wrenching moments are those that measure the impact of the young man’s quest on his family and friends.

Into the wild - Christopher McCandless

Hirsch looks uncannily like the real McCandless (whose final photograph we see at the end), and he effortlessly conveys every ounce of his intelligence, naivete, easy charm and self-destructive fascination for the wild that Krakauer so vividly described.

Penn’s direction is amazingly sharp and intuitive, full of masterful touches that give an epic dimension and scope to the parable. Scenes between McCandless and his various mentors are especially strong, and he gets performances from Vaughn, Keener and Holbrook that may be the best of their careers.

Pearl Jam’s Eddie Vedder supplied the songs for the soundtrack and I can’t think of a movie since “McCabe and Mrs. Miller” in which a particular artist’s songs have been used so effectively to establish mood, convey character and help tell a dramatic story. If one of them doesn’t win the best-song Oscar, there’s no justice.

For those who have read Thoreau’s Walden, there comes a time, maybe only lasting a few hours or a day, when the notion of living alone in a tiny cabin beside a pond and planting some beans seems strangely seductive. Certain young men, of which I was one, lecture patient girl friends about how such a life of purity and denial makes perfect sense. Christopher McCandless did not outgrow this phase.

Jon Krakauer’s Into the Wild, which I read with a fascinated dread, tells the story of a 20-year-old college graduate who cashes in his law school fund and, in the words of Mark Twain, lights out for the territory. He drives west until he can drive no farther, and then north into the Alaskan wilderness. He has a handful of books about survival and edible wild plants, and his model seems to be Jack London, although he should have devoted more attention to that author’s “To Build a Fire.”

Sean Penn’s spellbinding film adaptation of this book stays close to the source. We meet Christopher (Emile Hirsch) as an idealistic dreamer, in reaction against his proud parents (William Hurt and Marcia Gay Harden) and his bewildered sister (Jena Malone).

He had good grades at Emory; his future in law school was right there in his grasp. Why did he disappear from their lives, why was his car found abandoned, where was he, and why, why, why?

Into the wild - Christopher McCandless

He keeps journals in which he sees himself in the third person as a heroic loner, renouncing civilization, returning to the embrace of nature. In centuries past such men might have been saints, retreating to a cave or hidden hermitage, denying themselves all pleasures except subsistence. He sees himself not as homeless, but as a man freed from homes.

In the book, Krakauer traces his movements through the memories of people he encounters on his journey. It was an impressive reporting achievement to track them down, and Penn’s film affectionately embodies them in strong performances. These are people who take in the odd youth, feed him, shelter him, give him clothes, share their lives, mentor him and worry as he leaves to continue his quest, which seems to them, correctly, as doomed.

By now McCandless has renamed himself Alexander Supertramp. He is validated by his lifestyle choice. He meets such people as Rainey and Jan (Brian Dieker and Catherine Keener), leftover hippies still happily rejecting society, and Wayne (Vince Vaughn), a hard-drinking, friendly farmer. The most touching contact he makes is with Ron (Hal Holbrook), an older man who sees him clearly and with apprehension, and begins to think of him as a wayward grandson. Christopher lectures this man, who has seen it all, on what he is missing and asks him to follow him up a steep hillside to see the next horizon. Ron tries, before he admits he is no longer in condition.

And then McCandless disappears from the maps of memory, into unforgiving Alaska. Yes, it looks beautiful. It is all he dreamed of. He finds an abandoned bus where no bus should be and makes it his home. He tries hunting, not very successfully. He lives off the land, but the land is a zero-tolerance system. From his journals and other evidence, Penn reconstructs his final weeks. Emile Hirsch plays him in a hypnotic performance, turning skeletal, his eyes sinking into his skull while they still burn with zeal. It is great acting, and more than acting.

This is a reflective, regretful, serious film about a young man swept away by his uncompromising choices. Two of the more truthful statements in recent culture are that we need a little help from our friends, and that sometimes we must depend on the kindness of strangers. If you don’t know those two things and accept them, you will end up eventually in a bus of one kind or another. Sean Penn himself fiercely idealistic, uncompromising, a little less angry now, must have read the book and reflected that there, but for the grace of God, went he. The movie is so good partly because it means so much, I think, to its writer-director. It is a testament like the words that Christopher carved into planks in the wilderness.