"Wild" is the story of a very long walk, a trek of more than 1,000 miles along the Pacific Crest National Scenic Trail, from the Southern California desert to the lush forests of Oregon. Based on Cheryl Strayed’s best-selling memoir, the film begins, like the book, in the middle of the journey, with a scene that at once celebrates the sublimity of nature and the spirit of solitude and douses such romanticism in a demystifying spray of cold water.

Cheryl, played by Reese Witherspoon with grit, wit and unblinking honesty, reaches the top of a rocky outcropping with a panoramic view of surrounding mountains. Her panting, sweaty, irritated presence contrasts with the tranquil glory of the landscape, her contemplation of which is interrupted by the loss of a toenail and then of a hiking boot. That mishap elicits a keening howl of frustration and a perfectly self-defeating gesture of protest as she hurls the other boot away.

In due course, we will come to know a lot more about Cheryl — about her childhood and her sex life, about the death of her mother and the end of her marriage — but we start out with the crucial information that this is not a woman who makes things easy, for herself or anyone else. The story "Wild" has to tell is partly about how Cheryl deals with hardship, and about how, following a piece of long-ago maternal advice, she learns to put herself "in the way of beauty." But it is also about her appetite for difficulty and her insistence on confronting ugliness, inside and out.

Strayed’s book, written in a frank, funny voice that only occasionally drifts into clouds of self-help abstraction, is a complicated blend of grief memoir and travelogue. It tries very hard not to stick to the well-marked autobiographical path, blazed by St. Augustine in his "Confessions," from sin to salvation, even as it passes through some classically Augustinian moral territory. Before setting out on her trek, the author had been using heroin and cheating on her husband. She insists, however, that her goal is not redemption but self-acceptance, not a catalog of regrets but a clear view and welcoming embrace of experience in all its forms.

The structure of "Wild" is as complicated as its themes. The "action" on the trail — walking, thinking, pitching the tent at night and packing it up in the morning — is punctuated by looping reminiscences of the life that preceded it. What is most audacious about the film, directed by Jean-Marc Vallie ("Dallas Buyers Club") from a screenplay by Nick Hornby (yes, that Nick Hornby), is how closely it follows and how fully it respects Strayed’s free-associative, memory-driven narrative. In its thrilling disregard for the conventions of commercial cinematic storytelling, "Wild" reveals what some of us have long suspected: that plot is the enemy of truth, and that images and emotions can carry meaning more effectively than neatly packaged scenes or carefully scripted character arcs.

Vallie’s approach places enormous pressure on Witherspoon, who appears in nearly every frame of "Wild." The overstuffed backpack she hoists onto her shoulders is a metaphor not only for the baggage the character is carrying but also for the scale of the actress’s undertaking. It is clear from the beginning that her task is not to make us like Cheryl, but to help us understand her. And because Cheryl doesn’t understand herself — she can’t really say why she’s on the Pacific Crest Trail in the first place — a certain amount of patience will be required. We are not going to be charmed, teased, flattered or befriended. We will start walking and see what happens.

What happens is that a series of encounters in the wilderness — mostly with men, some of them scary, some of them kind, almost all of them intrigued by the presence of a solitary woman so far from civilization — dissolve into montages of memory. The dominant figure in these is Cheryl’s mother, Bobbi, an almost magical, not quite tragic figure played by Laura Dern. Bobbi’s death from cancer is the great trauma of her daughter’s life, and while the movie is careful not to posit grief as the cause of Cheryl’s reckless, risky behavior, it does lay out a row of dots for her and the audience to connect.

And it is Cheryl’s specific and ordinary suffering that connects us to her and allows us to share her adventure.

"Wild" the book is part of a literary tradition that stretches back to Thoreau and Wordsworth, and the movie is part of a more recent trend that includes "Into the Wild," "127 Hours" and "Tracks."

The paradox of all these accounts of lonely wandering is that they actively solicit the companionship of readers and viewers.

"Wild" may be full of natural beauty — Yves Bilanger’s cinematography unostentatiously captures the infinite colors and vistas of the West — but it is also a celebration of the power of art. Cheryl, a budding writer, is also a devoted reader, filling log books with quotations from Emily Dickinson, Ralph Waldo Emerson, Adrienne Rich and James A. Michener (the subject of a recollected argument with Bobbi). Those words crystallize some of the film’s ideas, while its feelings are carried along on a current of music: a well-timed Grateful Dead cover; hummed and whistled snatches of "El Condor Pasa," by Simon and Garfunkel; and an unexpected a cappella ballad that delivers a clean and overwhelming catharsis near the end.

"Wild" has its shortcomings. There is too much montage in the middle and too much voice-over at the end, and maybe not quite enough detail about some of Cheryl’s relationships (which is to say not enough Gaby Hoffmann, who plays her best friend). But you wouldn’t want a movie that celebrates imperfection, improvisation and the importance of mistakes to be slick or seamless. What makes its heroine worth caring about — what makes her a rare and exciting presence in contemporary American film — is not that she’s tidy or sensible or even especially nice. It’s that she’s free.

© 2014 The New York Times Company