Young tomboy Mattie Ross (Kim Darby) loves her father. But when this father gets killed by a tramp called Tom Chaney (Jeff Corey), she is determined to see him hang.She goes to Fort Smith and discovers that Tom Chaney has fled in the Indian territory, where only a marshall can get him. A marshall who has true grit, in order to vanquish not only Chaney, but also the group of outlaws he has connected with, led by the notorious bandit Ned Pepper (Robert Duvall) and including Moon (Dennis Hopper) and Quincy (Jeremy Slate).So Mattie finds the toughest marshall of the West : the one-eyed Reuben J. "Rooster" Cogburn (John Wayne), who's never known a dry day in his life, and hires him - with $100 - to go after Chaney and Pepper. Rooster soon pairs with young Texican La Boeuf (Glen Campbell) who also looks for Chaney as he had killed a Texas-senator before murdering Mattie's father.Mattie and La Boeuf are soon at odds, as the latest doesn't want her to join the chase, but Mattie hangs on and Rooster lets her come with them. Strong-headed Mattie, fast talking La Boeuf and sharp Rooster engage themselves in a battle of wits and, at the same time, on Chaney's tracks.After an ambush where Moon and Quincy are killed, Mattie gets face to face with Chaney and wounds him before being kidnapped by Ned Pepper. Ned forces Rooster and La Boeuf to withdraw and leaves Mattie with Chaney as a hostage. But La Boeuf comes back and captures Chaney.Mattie and La Boeuf witness the last fight between Ned Pepper and three of his men on one side, Rooster all alone on the other. They all charge on their horses and Rooster kills three men and injures Pepper before his horse (Bo) is killed under him. He desperately tries to get his gun as the dying Ned comes closer. At the very last moment, La Boeuf fires from the hill he stands on with Mattie and shoots Ned before Rooster gets killed.Chaney then escapes and hits La Boeuf on the head with a big stone, only to get shot again by Mattie. With the gunfire, she falls in a pit where a deadly snake threatens her. Rooster arrives at that moment, kills Chaney who was still trying to hurt Mattie and goes down the pit to save Mattie. The snake bites her and Rooster's unable to go up the pit with the unconscious Mattie. La Boeuf wakes up just a little while to pull them out with a horse and a rope, before falling dead. Rooster leaves him on the field and kills his horse under him to bring Mattie to a doctor.At the end of the movie, Mattie is back to her ranch where she heals slowly. Rooster comes and visits her. She shows him the cemetery where her father rests and she offers him, as he has no family, to rest beside her after his death. He takes on the offer with a joke and departs, jumping over a fence and yelling: "Come and see a fat old man sometime!"