Into the Wild recounts the true story of Christopher McCandless (Emile Hirsch), a student-athlete at Emory University, as told by his sympathetic sister, Carine McCandless (Jena Malone). In rejection of a materialist, conventional life, and of his parents Walt McCandless (William Hurt) and Billie McCandless (Marcia Gay Harden), who McCandless perceives as having betrayed him, McCandless destroys all of his credit cards and identification documents, donates $24,000 (nearly his entire savings) to Oxfam, and sets out on a cross-country drive in his well-used but reliable Datsun toward his ultimate goal: Alaska and, alone, to test himself and experience the wilds of nature. He does not tell his family what he is doing or where he is going and does not communicate with them thereafter, leaving them to become increasingly anxious and eventually desperate.
Along the way his automobile is caught in a flash flood and he abandons it to hitchhike after burning what remains of his dwindling cash supply at the side of Lake Mead, Arizona. He then creates a new name: Alexander Supertramp. Along his travels, he encounters a hippie couple Jan Burres (Catherine Keener) and Rainey (Brian H. Dierker). As McCandless continues his travels, he decides to work for a contract harvesting company owned by Wayne Westerberg (Vince Vaughn). However he is forced to leave after Westerberg is arrested for satellite piracy. McCandless then goes up at the Colorado River and when he is told that he may not go down by kayak without a license, he acquires a Perception Sundance 12 open-water kayak and, followed by the river police, paddles downriver eventually all the way into Mexico. There his kayak is lost in a sandstorm and he crosses back into the United States. Unable to easily hitchhike, he starts traveling via freight train to Los Angeles. Not long after arriving, however, he starts feeling "corrupted" by modern civilization and decides to leave. Later, McCandless is forced to switch his travelling method back to hitchhiking due to rough security.
McCandless then arrives at a hippie commune, Slab City and encounters Jan and Rainey again. At the commune, he meets Tracy Tatro (Kristen Stewart), who becomes attracted to McCandless. McCandless decides to continue his goal for Alaska, much to everyone's sadness. McCandless then encounters a retired but lonely leather worker, Ron Franz (Hal Holbrook) in Salton City, California. After spending several months with Franz, McCandless decides to leave for Alaska and Franz gives him gear to use. Franz offers to adopt McCandless as his grandchild, but McCandless tells him that they should discuss this after McCandless returns from Alaska and Franz becomes extremely saddened by his departure.
Nearly two years after leaving his family, McCandless crosses a stream in a remote area of Alaska and sets up camp in an abandoned Fairbanks Transit bus, the "Magic Bus", used as a shelter for moose hunters. Initially McCandless is exhilarated by the isolation, the beauty of nature around and the thrill of living off the land as the spring thaw arrives. He hunts and gathers, and reads books, and keeps a diary of his thoughts. However life becomes harder; his supplies start to run out and although he kills a moose the meat is spoiled by flies and maggots. He realizes that nature is also harsh and uncaring. Ultimately on his journey of self-discovery, he concludes that true happiness can also be found in sharing, and in the joy of realization seeks to return from the wild to his friends and family.
However, to his despair McCandless finds that the stream that he crossed has become a violent torrent and he cannot return; he is trapped by nature. He is forced to return to the Magic Bus but now as a prisoner; having previously insisted on being self-sufficient he is no longer in control of his fate and can only hope for help from the outside. As his supplies run out, he is forced to gather and eat roots and plants. He has a book to help him to distinguish edible from inedible, but he confuses similar plants and is poisoned. He slowly and painfully starves. In his final hours, he continues to document his process of self-realization and accepts his fate, as he imagines his friends and family for a final time.
The epilogue occurs two weeks after his death when his body is found by moose hunters. The movie ends with a picture of him, found undeveloped in his camera from before he died. It tells that his sister carried his ashes from Alaska to the eastern seaboard by plane with the ashes in her backpack.