In 1885, cattlemen Gil Carter and Art Croft travel from their small ranch to the nearby town of Bridger's Wells, Nevada, after the winter round-up. Gil is hoping to meet his sweetheart, Rose Mapen, and is infuriated when Darby, the bartender, informs him that she left town to be married. Gil's temper worsens when rancher Jeff Farnley insinuates that he and Art, as the only strangers present, may be responsible for the recent cattle rustling that has hit every rancher in the area. Gil and Farnley engage in a fistfight, which ends when Darby shatters a bottle over Gil's head. As Gil and Art are standing outside afterward, a rider rushes into the saloon. Gil and Art rejoin the crowd, which has just learned that Larry Kincaid, a well-respected local rancher, has been murdered, presumably by the rustlers. Farnley, Kincaid's best friend, is easily whipped into a frenzy by the town drunk, Monty Smith, and other bored men who insist that the perpetrators should be lynched. Storekeeper Arthur Davies tries to persuade the men to wait for Sheriff Risley and Judge Daniel Tyler, but when they persist in forming a posse, Davies sends Gil and townsman Joyce to get Tyler. Davies asks Gil to avoid involving Butch Mapes, the brutish deputy sheriff, but Mapes is at Tyler's house, and when he learns of the excitement, he joins the gathering crowd. Tyler tries to dissuade the men from pursuing the alleged criminals, but Smith, Farnley and the others insist that Tyler's justice moves too slowly. Smith caustically suggests that black preacher Sparks should come, and even though he knows Smith is kidding him, Sparks decides to go in case prayer is needed. The mob is joined by Jennie "Ma" Grier, a tough woman who also insists that they find Kincaid's killers. Tyler and Davies have almost persuaded the crowd to desist, however, when Major Tetley, a former Confederate soldier who now fancies himself a town leader, arrives and announces that three men were seen on Bridger's Pass, and that they had forty head of cattle bearing Kincaid's brand. Despite Tyler's protests that only Risley can appoint new deputies, Mapes swears in the posse members and they set off for the pass. Gil and Art reluctantly go along, for they fear that suspicion will fall on them if they do not participate. Gil's uneasiness about the situation increases when Sparks remarks that he still has nightmares about seeing his brother lynched many years previously. Night falls as the posse travels, and everyone begins to suffer from the cold. As they stop on the mountain road to rest, a stagecoach passes by and the driver mistakenly assumes that the crowd are robbers. Art is shot in the shoulder during the ensuing confusion, and while his wound is being cleaned, Gil discovers that the passengers are Rose, her new husband, Swanson, and his sister. After the wealthy Swanson vaguely warns Gil to stay away from Rose, the stage departs. Art is determined to stick with the posse, which continues on to the Ox-Bow Valley. There they find three sleeping men and the cattle bearing Kincaid's brand. After surrounding them, the mob awakens the three men, who are led by young rancher Donald Martin. Martin's companions are Alva Hardwick, an addled old man whom Martin calls "Dad," and a Mexican named Francisco Morez, who does not appear to speak English. Martin is amazed by Tetley's accusations and immediately protests their innocence. Martin insists that he moved to nearby Pike's Hole three days earlier and purchased the cattle from Kincaid, who was too busy to provide him with a bill of sale. Gil tries to persuade the others to bring the trio back to the judge, but Art reminds him that they may get lynched as well if they interfere. Davies also pleads for the men's lives, and finally, Tetley agrees to give them until dawn to prepare themselves. Martin writes a letter to his wife and two young children, while Dad sits in a daze and Morez hungrily consumes a meal prepared by Ma. While Davies tries to get Tetley to read Martin's moving letter, Morez attempts to escape. He is shot in the leg and brought back, and Kincaid's gun is found on him. Morez, who now reveals that he does speak English, asserts that he found the gun along the road, but the presence of the weapon seals his fate. Davies again protests the lynching, and this time, Sparks, Gil, Art, Tetley's cowardly son Gerald and two other men stand by him. They are outnumbered, however, and the condemned men are put on horseback. Tetley tries to force Gerald to whip the horse from underneath Martin, and when he cannot, Tetley knocks him unconscious. Martin, Dad and Morez are hanged, after which the now somber crowd leaves. Before they have journeyed far, though, they are joined by the sheriff, who tells them that not only is Kincaid alive, but his attackers have been caught. Risley promises that those responsible for the lynching will pay dearly, and the group rides back to town. There, Gerald castigates his father for his cruelty, and the distraught major commits suicide. Meanwhile, in the crowded saloon, a collection is taken up for Martin's wife. Gil and Art contribute, and Gil tries to get Art to read Martin's letter. Art cannot read, however, so Gil reads the letter aloud, and the men are ashamed to hear Martin's stirring words about the nature of justice and conscience. Gil and Art then leave Bridger's Wells on their way to deliver the letter and look after Martin's wife and children.