In 1880, Frank Ross (John Pickard) is murdered and robbed by his hired hand, Tom Chaney (Jeff Corey). Ross’s young daughter, Mattie (Kim Darby), travels to Fort Smith, Arkansas, where she hires aging U.S. Marshal Reuben “Rooster” J. Cogburn (John Wayne) to bring Chaney in, because Mattie’s heard that Cogburn has “true grit”. Mattie earns the money to pay his fee by shrewdly horse trading with Colonel Stonehill (Strother Martin). She gives him a payment to track and capture Chaney, who has taken up with outlaw “Lucky” Ned Pepper (Robert Duvall) in Indian Territory (modern-day Oklahoma).
A young Texas Ranger, La Boeuf (Glen Campbell), is also pursuing Chaney and joins forces with Cogburn, despite Mattie’s protest. The two try to ditch Mattie, but she catches up and is permitted to ride along.
After several days, the three discover horse thieves Emmett Quincy (Jeremy Slate) and Moon (Dennis Hopper), who are waiting for Ned Pepper at a remote dugout cabin. Cogburn captures and interrogates the two men. Moon’s leg is injured and Cogburn uses the injury as leverage to get information about Lucky Ned. In terrible pain and about to talk, Moon is stabbed by Quincy, who is then killed by Cogburn. In the remaining minute before Moon dies, he reveals that Pepper and his gang are due at the cabin that night to get fresh mounts.
Rooster and La Boeuf lay a trap. Upon arriving, Pepper is suspicious and succeeds in drawing La Boeuf’s fire, who blows their planned ambush by shooting and killing Pepper’s horse. A firefight ensues, during which Cogburn and La Boeuf kill two of the gang, but Pepper and the rest of his men escape unharmed. Cogburn, La Boeuf, and Mattie make their way to McAlester’s store (owned by J.J. McAlester, played by Stuart Randall) with the dead bodies. Cogburn tries to persuade Mattie to stay at McAlester’s, but she refuses.
The three resume their pursuit. Fetching water one morning, Mattie finds herself face-to-face with Chaney in a stream. When he comes toward her menacingly, she shoots Chaney with her father’s Colt Dragoon, injuring him and calling out to her partners. Pepper and his gang get there first, capturing her. Lucky Ned then forces Cogburn and La Boeuf to abandon the girl and ride away. Pepper decides to leave Mattie in the care of Chaney, who has lost his horse. He promises he will send a horse back for Chaney, vowing to kill him if any harm comes to the girl.
Cogburn doubles back and attacks Pepper and his gang single-handedly. La Boeuf, meantime, finds Mattie. They watch from a high bluff as a mounted Cogburn confronts Pepper’s gang. Cogburn gives Pepper a choice between being killed right there or surrendering and being hanged in Fort Smith. Calling this “bold talk for a one-eyed fat man”, Pepper enrages Cogburn, who charges the four outlaws, guns blazing. In the initial head-on charge, neither side hits the other. Cogburn eventually kills two of the gang, hits Pepper repeatedly, with one gangster fleeing. In the fight, Ned shoots Rooster’s horse, trapping Rooster’s leg under him as he goes down. As a last act, the mortally wounded Pepper prepares to kill Rooster, until La Boeuf makes a long shot with his Sharps rifle, blowing Ned out of the saddle and killing him.
As La Boeuf and Mattie return to Pepper’s camp, Chaney comes out from behind a tree and strikes La Boeuf in the head with a rock, fracturing his skull and knocking him unconscious. Mattie is able to shoot Chaney and wound him, but driven back by the recoil, falls into a snake pit and breaks her arm. Chaney begins to taunt Mattie about the snakes; Cogburn appears and shoots Chaney, killing him. The dead Chaney then falls into the pit as well. With great difficulty, Cogburn descends into the pit on a rope to retrieve Mattie, who is bitten by a rattlesnake before Cogburn can kill it. The mortally injured La Boeuf helps them out of the pit, saving their lives. La Boeuf dies from the effort.
Cogburn is forced to leave La Boeuf’s body behind as they race to get help for Mattie at McAlester’s on Mattie’s pony, which dies while carrying them. After stealing a buckboard, they arrive at their destination. There, an Indian doctor treats Mattie’s snakebite and broken arm.
Sometime later, Mattie’s attorney, J. Noble Daggett, (John Fiedler) meets Cogburn in Fort Smith. On Mattie’s behalf, Daggett pays Cogburn for his part in Chaney’s capture, plus a bonus for saving her life. Cogburn offers to wager the money on a bet that Mattie will recover just fine, a bet Daggett declines.
In the epilogue, Mattie, her arm in a sling, is back at home recovering from her injuries. She promises Cogburn he will be buried next to her in the Ross family plot after his death. Cogburn reluctantly accepts her offer and leaves, jumping over a fence on his new horse to disprove her good-natured jab that he was too old and fat to clear a four-rail fence, and rides off into the valley below.