The father of 14-year old Mattie Ross (Hailee Steinfeld) is murdered by hired hand Tom Chaney (Josh Brolin) while on a trip to Fort Smith, Arkansas to purchase horses. While collecting her father's body, Mattie asks the local sheriff about the search for Chaney. He tells her that Chaney has, likely, fled with "Lucky" Ned Pepper (Barry Pepper) and his gang into Indian Territory, where the sheriff has no authority, so she inquires about hiring a Deputy U.S. Marshal. The sheriff gives three recommendations, and Mattie chooses Rooster Cogburn (Jeff Bridges). Cogburn initially rebuffs her offer, disbelieving both her grit and her wealth, but she raises the money by aggressively horse-trading with Colonel Stonehill.
Texas Ranger LaBoeuf (Matt Damon) arrives in town, pursuing Chaney for the murder of a Texas State Senator. LaBoeuf proposes joining Cogburn, but Mattie refuses his offer. She wishes Chaney to be hanged in Arkansas for her father's murder, not in Texas for killing the senator. Mattie insists on traveling with Cogburn but he departs without her, having gone with LaBoeuf to apprehend Chaney and split the reward.
After being refused passage on the ferry that conveyed Cogburn and LaBoeuf, Mattie fearlessly goads her new horse, Blackie, into swimming the river with her. After she successfully crosses, LaBoeuf expresses his displeasure at her obstinance by attempting a spanking with a stick. Cogburn forces him to stop and eventually allows Mattie to accompany them. After a dispute over their respective service with the Confederate Army, Cogburn ends their arrangement and LaBoeuf leaves to pursue Chaney on his own. Cogburn and Mattie meet a trail doctor who directs them to a supposedly empty dugout for shelter. At the dugout, they find two outlaws, Quincy (Paul Rae) and Moon (Domhnall Gleeson), likely waiting for the rest of the Pepper gang. After literally 'smoking' the two out, Cogburn and Mattie capture and interrogate them. Quincy insists they have no information about the Pepper gang. Eventually, however, the injured Moon, seems to be about to divulge what he knows whereupon Quincy stabs him in the chest, and Cogburn shoots Quincy dead. Before dying, Moon asks that his body be relayed to his brother and says Pepper and his gang will be returning for fresh horses that night.
Cogburn and Mattie wait on the hillside above the dugout for the arrival of Ned Pepper and his gang. However, LaBoeuf arrives first and then is confronted by the arrival of the Pepper gang. Cogburn shoots two gang members and accidentally hits LaBoeuf, but Pepper escapes. LaBoeuf is injured in the shoulder and in the fall from his horse nearly bites his own tongue off.
