Mattie Ross acknowledges that most people don’t believe a fourteen-year-old girl is capable of avenging her father’s death. Nonetheless, this is what she does after her father’s murder. Planning to buy a group of ponies from a man named Stonehill, her father traveled from Little Rock to Fort Smith, Arkansas to make the deal. While Mattie stayed at home with her mother and two younger siblings, Frank Ross took along Tom Chaney, a man from Louisiana who appeared one day and asked for work. Taking pity on Chaney—a seemingly luckless man—Frank agreed to give him a job. When they arrived in Fort Smith, though, Chaney acted wildly by gambling his money away. Afterwards, he flew into a rage, got drunk, and decided to go after the men who won his money. As he vowed to steal back his money, Frank tried to stop him. Chaney shot him and stole two gold pieces from him. He then ran to Stonehill’s stables, where he clubbed a watchman in the face and made off with Frank’s horse, Judy.
After her father’s death, Mattie travels to Fort Smith and talks to the sheriff, discovering that the police are hardly doing anything to catch Chaney, who has run off with a notorious bandit named Lucky Ned Pepper and his gang, a group that recently robbed a mail train. The police believe Chaney and the gang have escaped into Indian Territory, where they have no jurisdiction. As such, Mattie decides to hire Rooster Cogburn, a marshal the sheriff says is ruthless. He also tells her that Rooster will be in court the following morning. Mattie then tells Yarnell—her father’s employee who accompanied her to Fort Smith—to travel back to Little Rock with Frank’s body while she stays in Fort Smith.
That night, she stays at the same boarding house where her father and Chaney stayed. The next day, she visits Stonehill and asks him to buy back the ponies, but he refuses, saying the deal has already been made. Mattie threatens to bring him to court, but he doesn’t take her threat seriously. Eventually he realizes that she and her lawyer—Lawyer Daggett—will build a strong case, and after much haggling, Mattie convinces him to buy back the ponies and to pay for the horse that Chaney stole, since it was on Stonehill’s property when Chaney took it. Begrudgingly, Stonehill agrees to pay her $325 as soon as she produces a letter from Daggett outlining the terms of the deal.
Mattie goes to the “telegraph office” and sends a message to Daggett. Later, she goes to the courthouse and watches Rooster Cogburn testify against Odus Wharton, the last surviving man of criminal family. Rooster has killed so many Whartons that Odus’s lawyer portrays him as unnecessarily violent. While he’s on the stand, Rooster explains that he and his longtime partner, Potter, came upon the last three Wharton men shortly after the criminals murdered a man and robbed him. According to Rooster, he and Potter approached with their guns drawn, but the Whartons attacked, so they had to shoot. Potter was killed in the fight. As for the Whartons, Rooster killed two of them and brought Odus back to Fort Smith. Eventually, the judge decides to continue the hearing the next day, giving Mattie a chance to catch up to Rooster, and she asks him to hunt down Chaney. Before accepting the offer, Rooster invites her to have dinner.
Over dinner, Mattie explains the details of the case. Rooster is familiar with Lucky Ned Pepper and is tempted to take the case, but he remains hesitant, since he isn’t sure if Mattie really has money. Later that night, Mattie goes back to the boarding house, where she remains for several days because she falls ill. Just when she’s starting to feel better, a Texas Ranger named LaBoeuf moves into the boarding house and tells her that he too is looking for Chaney. He explains that Chaney is only a fake name for a criminal named Theron Chelmsford, whom LaBoeuf has been tracking ever since he killed a senator and his dog in Waco, Texas several months ago. Glad to hear that someone else wants to find Chaney, Mattie informs LaBoeuf that she’s hiring Rooster, and LaBoeuf considers joining forces. However, it soon becomes clear that LaBoeuf wants to bring Chaney back to Texas, which Mattie dislikes, since she wants her father’s killer to hang in Fort Smith. Their conversation ends in an argument.
Lawyer Daggett’s letter arrives, so Mattie closes the deal with Stonehill. She then visits Rooster, gives him a down payment, and informs him that she’ll be coming with him. He rejects this at first but soon sees she won’t be deterred. The next time Mattie sees him, though, he’s sitting with LaBoeuf, who says he can make more money if he helps bring Chaney to Texas, since the senator’s family has promised a large reward. Rooster switches sides, teaming up with LaBoeuf instead of helping Mattie.
Returning to Stonehill, Mattie buys back one of the ponies (whom she names Blackie) and sets out early the next morning, knowing Rooster and LaBoeuf are planning on beginning their journey on a small ferry. When she boards the boat on Blackie, though, Rooster and LaBoeuf instruct a deckhand to escort her back onto shore, at which point the ferry pulls away. The deckhand leads her to the top of a hill, but she tricks him and rides away, steering her horse through a narrow part of the river and thrashing through the water until she’s on the other side. Seeing this, LaBoeuf and Rooster gallop away, but she chases them until they suddenly stop and take her off Blackie, at which point LaBoeuf whips her with a switch. After a while, Rooster takes pity and orders LaBoeuf to stop, but LaBoeuf doesn’t listen, so Rooster pulls out his gun, and the Texas Ranger finally relents.
As Rooster, LaBoeuf, and Mattie venture on, they learn that Lucky Ned Pepper and two other outlaws were seen at a store called McAlester’s three days before. After a day of hard riding and a night on the uncomfortable ground, the trio comes upon a manmade dugout the following evening, where they find two criminals acquainted with Ned Pepper. Their names are Quincy and Moon, and Rooster ends up shooting Moon in the leg in order to get the two to cooperate. As Moon slowly bleeds out, he tells the trio that they saw Ned and his partner Haze two days before. To quiet him, Quincy cuts off four of his fingers, at which point LaBoeuf shoots Quincy in the neck, killing him. Before Moon dies, he says that Ned and Haze are planning to come back to the dugout sometime that night to eat dinner and pick up a group of horses that Moon and Quincy stole, which are currently standing out in the woods. Moon dies, and Rooster instructs Mattie and LaBoeuf to tidy up the dugout and restoke the fire so that it looks like there are people inside. The trio then splits up, with LaBoeuf hiking up one ridge and Rooster and Mattie hiking up another, waiting for Ned and his group to return. The plan, Rooster explains, is to trap them in the dugout, which is set into a V-like formation between the ridges. When the outlaws finally arrive hours later, though, Ned shoots his gun in the air, and this startles LaBoeuf, who returns fire, missing Ned but hitting his horse. As the outlaws scatter, Rooster and LaBoeuf fire at them, killing Haze and a younger criminal while the others escape. LaBoeuf sustains a minor injury to the shoulder when one of the bandits shoots at him and shatters the stock of his rifle.
As the sun rises, the trio makes its way to McAlester’s store with the bandits’ horses and the two dead criminals. From there, they set out once again, riding for an incredibly long time as Rooster drunkenly leads them to where he thinks the bandits have gone. After the longest day of Mattie’s life, they finally go to sleep in an area Rooster claims is four miles from Ned Pepper’s gang. The next morning, though, Mattie finds herself face-to-face with Tom Chaney when she goes to get water from a stream. She confronts him but he doesn’t take her seriously, even after she turns her father’s old pistol on him. Just as he’s telling her to come with him, she shoots him in the stomach. Her next shot misfires, and Chaney manages to grab her and bring her up the other side of the stream before LaBoeuf and Rooster can save her. A shootout takes place between the two groups, but Rooster and LaBoeuf are forced to retreat, shouting to Ned that they’ll leave as long as he doesn’t hurt Mattie.
Ned takes Mattie to the bandits’ camp atop a small hill. Before long, they see Rooster and LaBoeuf riding away, and they begin to relax, even divvying up their shares from their most recent train robbery. Once this is done, Ned decides everybody will leave except for Chaney and Mattie, since Chaney’s horse got away when Mattie shot him. Although Chaney feels betrayed, Ned tells him to take Mattie to a certain place, where he can drop her off and secure a horse for himself. With this, he and his cronies ride away. After a moment, Mattie throws a pot of hot water on Chaney and starts to run, but Chaney catches her and hits her on the head with his pistol. Just then, LaBoeuf appears and holds Chaney at gun point, having snuck up the backside of the hill. Looking down, Mattie sees Ned Pepper and his gang riding on the plain, where Rooster suddenly appears and charges them at full speed, shooting while holding the reins with his teeth. He manages to best everyone but Ned himself before toppling over, his horse falling atop him. Ned approaches Rooster and prepares to kill him, but LaBoeuf takes aim from the hill and shoots Ned where he stands.
Mattie and LaBoeuf celebrate this fantastic shot, but Chaney clubs LaBoeuf over the head with a rock, sending him to the ground. Mattie grabs her gun and shoots Chaney in the head, but the force of the shot sends her toppling backward, and she falls into a large pit that Chaney previously claimed was full of rattlesnakes, though she doesn’t see any. Still, she discovers that her arm is broken and that the bottom half of her body is lodged in the mouth of a bat cave that extends even farther down. Just as she’s about to fall through completely, she hears Chaney’s voice and realizes he’s still alive, though his body comes flying down into the pit when Rooster appears several moments later. When Chaney’s body hits the ground, he crushes an old human skeleton lying nearby—a skeleton full of hibernating rattlesnakes, which suddenly slither all around Mattie, eventually biting her in the hand. Just before she’s about to lose consciousness, Rooster propels himself down and grabs her, and LaBoeuf hoists them out. From there, Rooster sets off with Mattie, riding Blackie back to Fort Smith while LaBoeuf stays with Chaney’s body. Before they reach safety, though, Blackie dies of exhaustion, and so Rooster takes Mattie in his arms and runs.
Mattie spends a week at the doctor’s, where they’re forced to amputate her arm because of the snake venom. In the years after this adventure, Mattie hopes to see Rooster again but never manages to catch up with him, as he travels around the nation taking up odd jobs. Finally, she hears decades later that he’s joined a traveling circus, but when she goes to see him, she learns that he died two days earlier.