In 1878, a determined 14-year-old Mattie Ross travels to Fort Smith, Arkansas, after her father is murdered by the hired hand Tom Chaney. She is sent to collect his body and quickly learns that Chaney has likely fled with Ned Pepper and his gang into Indian Territory, where local authority fades away. Mattie begins to explore justice as she looks for a deputy U.S. Marshal, and the sheriff presents three possibilities. She picks the one described as the meanest, Rooster Cogburn, who initially rebuffs her, doubting both her grit and her wealth, but she compensates by bartering aggressively for horses and gathering the money herself.
Texas Ranger LaBoeuf arrives in town, pursuing Chaney for the murder of a state senator. He suggests joining Cogburn on the hunt, but Mattie declines his offer because she wants Chaney hanged in Arkansas, not Texas. She insists on traveling with Cogburn to witness justice, yet Cogburn departs with LaBoeuf to chase Chaney and split the reward. The trio’s uneasy alliance tests loyalties and temper as they push deeper into a pursuit that crosses state lines and moral lines alike.
After catching up with the lawmen, Mattie endures a rebuke from LaBoeuf for her insolence, a moment that escalates until Cogburn draws his weapon on LaBoeuf. The clash over authority and their conflicting views of the veteran guerrilla fighter William Quantrill set the tone for Cogburn’s decision to end his arrangement with LaBoeuf, who heads off to pursue Chaney on his own. At a rural dugout, Cogburn and Mattie encounter outlaws Moon and Quincy, who surrender after Cogburn shoots and injures Moon. The outlaws initially deny knowing Ned Pepper or Chaney, but Cogburn leverages Moon’s worsening condition to press for information. Quincy, enraged, stabs Moon and is himself felled by Cogburn. A dying Moon reveals that Pepper’s gang will soon descend on the dugout for supplies.
Cogburn and Mattie plan an ambush, but LaBoeuf arrives first and engages the gang, getting caught in the crossfire. Cogburn shoots two gang members and accidentally hits LaBoeuf, though Pepper escapes. With LaBoeuf gravely wounded, he reluctantly re-joins the pursuit, their bond strained but intact. The trio presses on, convinced that Chaney and Pepper’s gang may be hiding in the rugged Winding Stair Mountains. Cogburn’s drinking grows heavier, and the tension between him and LaBoeuf resurfaces as days pass without a clear trail.
The trail goes cold, Cogburn proclaims, and he quits the pursuit. LaBoeuf returns to Texas while Mattie and Cogburn reconcile the earlier disagreements, acknowledging that they misread each other’s resolve. While grabbing water from a stream, Mattie encounters Chaney and manages to wound him, but her revolver misfires, allowing Chaney to seize her as a hostage. Ned Pepper’s gang uses the threat to compel Cogburn to withdraw by promising to kill Mattie if Cogburn stays. Chaney, thinking Pepper has abandoned him to capture, contemplates killing Mattie, while LaBoeuf arrives and knocks Chaney unconscious, revealing that Cogburn and LaBoeuf had reunited shortly after the initial gunfight. LaBoeuf had intended to rescue Mattie while Cogburn intercepted the gang in a tense four-on-one standoff.
The assault charges forward as Cogburn and the outlaws collide in deadly tempo. Cogburn kills two gang members, and a third flees after his horse is struck. Pepper, mortally wounded, presses on with resolve as Mattie’s hostage situation escalates. LaBoeuf, firing from long range, ends Pepper’s life with a precise shot from about 400 yards, though Chaney regains consciousness and unexpectedly knocks out LaBoeuf. Mattie seizes the rifle and shoots Chaney dead, the recoil sending her into a rattlesnake den where a rattlesnake bites her hand. Cogburn springs into action, kills the snakes, and drags Mattie to safety, thanking LaBoeuf and promising to arrange help, then rides on with Mattie toward a doctor. Exhausted and with a failing horse, Cogburn shoots the horse to spare Mattie the burden and carries her on foot until help arrives. He remains by her side until she’s out of immediate danger, but he disappears before she wakes, leaving her to learn that her arm will be amputated.
A quarter century passes before Mattie receives a letter inviting her to a traveling Wild West show in which Cogburn is performing. When she finally reaches the show’s location, she learns from Cole Younger and Frank James that Cogburn had died three days earlier. She thanks Younger with quiet gratitude, remarking on James’s lack of courtesy, while her own life remains single. Mattie has Cogburn’s body moved to her family cemetery in Yell County, Arkansas, and she contemplates visiting LaBoeuf, wondering if the man she once met remains the same, or if time has changed him as much as it did the world around them.
