In the opening scene (set in the year 1995), a young woman named Cheryl Strayed (Reese Witherspoon) is seen reaching the top of a cliff while carrying a big pack. She is exhausted and takes a seat, where she removes one of her boots. She peels off the sock to reveal that one of her toenails is about to fall off from all the pressure on her feet. She peels the toenail off, but then accidentally knocks the boot off the cliff edge and watches helplessly as it tumbles down the cliff face. She removes the other boot and hurls it over the edge, screaming in frustration.
In a flashback, the movie goes back to the first day of her journey where she checks in to a motel room with an enormous backpack. The motel clerk tells her that there will be an extra charge if a man stays with her, and though Cheryl insists she is by herself, the clerk keeps repeating there will be an extra charge. The clerk also asks for a home address. Cheryl explains that she's hiking the Pacific Crest Trail (PCT) and doesn't have an address, so the clerk tells her to put her parents' address down. Cheryl hesitates but eventually puts an address down.
She goes to her room and makes a phone call to a man. She tells the man that she had to put an address down for her motel and she provided his. The man, Paul (Thomas Sadoski), confirms that it's fine and she should feel free to put his address down. Cheryl says she was also looking for her brother, even though she's not sure he will even care. Paul tells her that he will pass along a message to her brother and asks how she's feeling about the hike. They have a strained conversation and eventually Paul tells her that he has company and he's making dinner so he will need to get off the phone. Cheryl seems to be uncomfortable with this information but Paul tells her that she's the one who felt she had to hike 1,000 miles.
As Cheryl goes through her prep, we see flashes of memories, Cheryl having sex with someone, her mother dancing, Cheryl using heroin, Cheryl fighting with a man in a car. These flashes happen repeatedly throughout the movie. Her pack is enormous and she can barely get it on her back and stand up with it. She goes outside to a gas station to find someone to approach for a ride. She sees an older man traveling with a woman in a minivan, and they agree to give her a ride to the beginning of the trail. The man plays oldies on his radio, which Cheryl sings along to and remembers singing along with her mother as a child.
The couple drops Cheryl off at the beginning of the trail, which is marked by a signpost as well as a notebook log for travelers to sign (Day 1). She leaves quotes and song lyrics along the path, attributing them to both the original speaker as well as herself. She looks ahead with uncertainty, but forges ahead. She only makes it a few miles before stopping to set up a camp. She is clearly a novice hiker and camper. During her first stop, she struggles to put up her tent. When she gets out her camping stove, she realizes that she purchased the wrong kind of gas. She adds water to her grains and just eats the mush. She continues for several days, at a slow pace, eating nothing but mush. She comes to a literal crossroad, and she can continue on the trail or take the paved road back to a town. She decides to keep going.
One afternoon (Day 5) she finally comes upon a truck and someone riding a tractor. She runs up to catch the rider before he rides away to beg for a ride back to town. As she approaches, she sees it's a man by himself. She asks if he could give her a ride somewhere she can get a hot meal. The man seems suspicious of her and says he's working, and she says she can wait for him to be done. He says by the time he's done, no restaurant in town will be open. She responds that she can camp somewhere near the restaurant and wait for it to open. Realizing that she won't be put off, the man agrees to take her when he's done and tells her to wait in his truck.
Cheryl waits in the truck and pokes around while waiting for the man. She finds a gun in the glove compartment, which she puts back. The man gets in the truck when he's done, sitting and starring at her. Cheryl starts to feel uncomfortable. The man tells her that he's decided that she can go to his house where she can have a hot meal and a shower. Cheryl tells him that he doesn't have to do that, he can just take her into town, but he insists. He also swigs from a flask and hands it to her. She takes it and feeling more and more uncomfortable, she tries to casually tell him that her husband had hiked on ahead of her and they had planned to regroup in a few days. The man stares at her and then tells her he likes to relax after a hard day's work. The man reaches under his dashboard and yanks something out, revealing it to be red licorice. He grins and hands Cheryl a piece, telling her not to tell his wife as she doesn't allow him to have candy. Cheryl laughs and the man drives her to his home. The man and his wife welcome Cheryl into their home, serving her a hot home-cooked meal, though the wife puts down newspapers on Cheryl's chair as she's been hiking for many days. The man remarks that if he wouldn't allow his wife to go on such a difficult hike on her own and his wife warmly teases him. Despite their rough appearance, they are clearly kind, decent people.
In the morning, the man drives Cheryl into town so she can buy the right kind of gas for her stove and takes her back to the trail. Cheryl confesses that she made up the story about her husband because she was nervous, but the man tells her that he understands. He asks her if she ever thinks about quitting, and she tells him that it only occurs to her every two minutes. He tells her that he's quit many things in life, marriages, jobs, but he never felt he had a choice.
Cheryl continues on the trail slowly. As she walks in solitude, there are multiple flashbacks which unfold out of order. She remembers her mother, Bobbi (Laura Dern), dancing with her and her younger brother Leif (Keene McRae) as children. She remembers her mother rushing them out of their childhood home to escape her abusive father. She remembers the rural farmhouse they moved to in rural Minnesota, where they lived in poverty as her mother tried to make their childhoods happy and full of love.
Cheryl reads a book in her tent at night, and in a flashback we see that she derided her mother for reading the same book, dismissing the author and his writing style. Apparently Bobbi decided to go back to college late in life and attended the same college as Cheryl, at the same time. Cheryl remembers being embarrassed of her mother, barely acknowledging her in a hallway, but then apologizing to her for it later that night. Bobbi tells her she understands that it's weird, and it's fine. Leif arrives home with a friend, and Bobbi immediately stops her homework to fix them something to eat. Cheryl is annoyed by this and tells Bobbi and Leif is old enough to get himself something to eat and that Bobbi should be focused on her schoolwork, but Bobbi tells her that being a mother is the most important thing in her life.
Cheryl continues to hike (Day 10), struggling with her large pack and encounters a rattlesnake, rainstorms and other struggles. On Day 12, she eventually comes upon another hiker bathing in a stream named Greg (Kevin Rankin). They have a brief, friendly chat about the hike and how it's going. Greg tells her that he's been averaging over 20 miles a day, and Cheryl is embarrassed to admit how slowly she's been going. Greg also tells her that a section of the trail was hit by snow and that most hikers will be taking a bus to bypass the section. They make plans to meet at Kennedy Meadows, a major stopping and resupply point on the trail, which Greg will reach in just a few days.
Cheryl continues on the hike, finally reaching Kennedy Meadows (on Day 14), where she is greeted enthusiastically with applause by Greg and several other hikers, who are all men. Greg offers to buy Cheryl a drink, and she tells him that she's been dreaming of Snapple lemonade and potato chips for days. The hikers are all impressed with Cheryl, but they tease her about her enormous pack, which they've nicknamed Monster. Cheryl picks up a package that was sent by her friend Aimee (Gaby Hoffmann) with fresh clothes and more supplies. A man named Ed, who works at the campsite, offers to help Cheryl repack her gear to lighten the load. He gently teases her about the many items she's packed that she realizes she's never used like heavy camera equipment, condoms and other items. He also advises her to stop carrying so many books, but she refuses to dump them. He tells her that she should rip out the pages of a book after she's read that section to lighten the weight. When she mentions that she's been losing toenails, he advises her that it means her shoes are too small and she should call REI who will send a new pair to the next pick up station where they will be waiting for her. Greg again advises her to bypass the snow covered section and take a bus, but Cheryl insists she wants to hit her 1,000 mile goal. He simply tells her to add extra distance to her originally planned ending point and finish up at the Bridge of the Gods in Oregon instead.
Cheryl is next seen taking a bus from Kennedy Meadows to a point around the mountains near Reno, Nevada to continue the hike. One day (Day 25), while hitchhiking along a road, she has an encounter with a man driving by who introduces himself as Jimmy Carter who asks her some questions saying that he works for a magazine called Hobo Times, and assumes that she is a hobo. Despite telling him that she is just hiking the PCT, be becomes pushy and asks her questions such as her current place of residence as well as job status. Cheryl becomes annoyed with him and tells him to leave, and he drives off.
After hitching another ride with a hippie family, Cheryl continues on the hike, struggling with the difficult path and bad conditions. She has to cross rivers and hike through snow up in the Sierra Mountains, but perseveres on. She has an encounter with two snowboarders whom she asks for directions. One of them tells her that she is current in Plumas County, California. They ask if she is lost, but she tells them 'no' and continues on. While making camp, she sees a red fox looking at her. It runs off when she approaches it.
She reaches the point shown in the opening scene, where she loses her boots down a cliff face. She straps on her sandals, duct taping them to her feet for extra support, and continues on.
On Day 49, when she reaches the next stopping point on the trail, she is relieved to find her new boots waiting for her. She is also overjoyed to meet another woman hiking the trail, as there are so few of them. The woman, named Stacey Johnson, tells her that she met Greg along the way who ultimately had to quit the hike before completing it. Cheryl is astonished to hear that she is still hiking when Greg, the experienced hiker, had to quit.
In a flashback, Cheryl remembers complaining to Bobbi about the state of their lives. They both work full time as waitresses and live in a dump of a house. She asks Bobbi if she's not unhappy with her life, having been married to "an abusive asshole". Bobbi tells her that she doesn't regret being married to an abusive man because it brought her children into her life.
More flashbacks show that Bobbi is diagnosed with a late stage spinal cancer. She succumbs much more quickly than expected in March 1991 at age 47, and it sends Cheryl into a tailspin. She and Leif are further traumatized by having to put her mother's beloved horse down by shooting it. Cheryl begins to withdraw from life; she begins having sex with random strangers.... in a hotel room... in the alley behind the diner where she works... while still going home to her husband Paul. She also sleeps with a man who introduces her to heroin. Paul finds her in a drug den one day, sleeping naked with the man. He brings her home where after an argument about her current status, they agree to divorce.
As Cheryl hikes across the deserts of northern California, she reaches a water station (on Day 56), desperate to refill her bottles. However, the water has run out and Cheryl is left with nothing to drink. She hikes along and finds a puddle of stagnant water filled with mosquitoes. She gets out her water filtration pump, fills it, adds iodine and waits for the water to be treated and safe to drink. As she sits, two hunters come upon her who are also looking for water. She explains that she's waiting for her water to be treated, but they are free to use her pump. One of the hunters makes sexually suggestive comments to her and leers at her, so Cheryl says that she will be on her way. She packs up her things, and the hunters say they will be leaving as well and head back the way they came. Cheryl walks off for a bit, but then returns to the site when the hunters are out of view. She changes her clothes and starts to set up her tent for the night when one of the hunters suddenly reappears. It is the same creep who leered at her, and he tells her that he likes her new clothes better because they show off her figure. He asks her why she came back and if she had been lying to them about leaving to get rid of them. She nervously tells him no, she just changed her mind. The tense moment is broken up when the other hunter calls for the man to hurry up and get back. The creepy hunter leaves and Cheryl quickly packs up and runs away as fast as she can.
In another set of flashbacks, we see Cheryl getting matching tattoos with Paul. The tattoo artist asks what the occasion is, and Paul casually says that they are getting divorced but wanted to commemorate their journey together. The tattoo artist seems surprised, particularly when Cheryl blurts out that she cheated, a lot. The tattoo artists says that Cheryl seems sorry, and she responds that she is, nearly in tears. Paul soothes her. As they are about to mail their divorce papers, Paul asks how Cheryl's feeling about the new name she's chosen for herself, Strayed. Cheryl says it feels appropriate, given what's happened in her life. They mail the papers, embrace and separate.
On the trail again, (on Day 62) Cheryl crosses the Oregon state like comes to the small town of Ashland which filled with hikers and many hippie types. She goes into a makeup store and samples products when the saleswoman tells her that no matter how much lipstick she puts on, she must take care of her personal hygiene first. Cheryl leaves the store and encounters an attractive man who is handling out flyers for a concert. He approaches Cheryl, who tries to maintain her distance. He gives her a flyer and tells her that he will put her name on the guest list for the event.
Cheryl goes to a motel for the night, showers and makes herself up, heading to the show. The man brings her a drink and teases her about being unrecognizable now that she's all cleaned up. She goes home with him and sleeps with him. In the morning, she makes a phone call to Paul, leaving him a message. She tells him that she wrote his name in the sand on a beach that morning, as she does every time she's on a beach, but she won't do it anymore.
Another few days later, Cheryl hikes through the rain to reach another supply station at Mount Hood State Park, but the man who works there is closing up for the day. She begs him to reopen so she can get her package which will contain fresh clothes and more supplies. The man says he will reopen for her, if she promises to have a drink with him. She hesitantly agrees, and he reopens the mail office. As he is inside getting her package, three young men hikers also enter the building. They recognize Cheryl as being the only woman who has been signing the mile marker logs and repeat back many of the quotes and lyrics she has left behind. The man comes out with Cheryl's package but tells the young men that he's already reopened once, and he won't do it again. Cheryl points out that he's not reopening because it's still open, and that given the miserable weather conditions, the packages will bring a little bit of relief. The man begrudgingly retrieves the other packages. Cheryl's package contains a letter from Paul who tells her that he's so proud of her for making it this far and he hopes she finds what she's looking for.
Cheryl enjoys hanging out with the young hikers that night as they all camp out. In the morning, the man brings Cheryl a cup of coffee and a doughnut, which she gratefully accepts and the young men tease her over.
In another flashback, Cheryl thinks of her friend Aimee who has been sending the packages for her. Cheryl meets Aimee for coffee who tells her she's disappointed in her for her behavior, using heroin and sleeping around. Cheryl defiantly tells her that she will try anything once and she's not ashamed of it. However, she breaks down when she confesses to Aimee that she thinks she's pregnant and unsure of who the father is. Aimee forces her to go to a store to buy a pregnancy test, and as they wait in line to purchase it, Cheryl sees a book about the Pacific Crest Trail. She takes the test, which confirms she is pregnant. She breaks down and tells Aimee that she can't be somebody's mother and decides to get an abortion right away. She grieves for her own mother and decides that she will go on a solitary hike on the PCT to walk herself back into the woman her mother raised her to be rather than the mess of a woman she's become.
On Day 80, Cheryl is hiking through the woods when she encounters a lama, and an older woman and a young boy running up to her after their pack lama ran way. The little boy asks Cheryl about herself and she tells him about her father who left her and her mother who recently died. The boy sings Cheryl a song which makes her cry.
She continues on the trail and finally reaches the Bridge of the Gods (on Day 94). In a final voice-over, as she walks across the bridge, Cheryl talks about finally finishing the hike, and ultimately remarrying and having two children of her own. She ponders whether she actually regrets the mistakes she's made, as it brought her to the place she is in her life now.
Over the closing credits, there are photos of the real Cheryl Strayed taken during her real-life hiking journey.