In the small bordertown of Rio Bravo, Sheriff John T. Chance (John Wayne) takes action by arresting Joe Burdette (Claude Akins) for the brutal murder of an unarmed man. However, the situation takes a dangerous turn when Joe's older brother Nathan (John Russell), the most powerful rancher on the border, orders his riders to blockade the town, preventing Chance from getting his prisoner to the U.S. marshal or seeking assistance.
As professional gunmen hired by Burdette silently infiltrate the town, meticulously watching Chance's every move, the sheriff finds himself with limited support. His only allies are his regular deputy, the aged and crippled Stumpy (Walter Brennan), and the newly sworn-in Dude (Dean Martin), a former deputy and renowned gunslinger attempting to overcome a two-year drunk to aid his friend. Rio Bravo becomes a temporary haven for Feathers (Angie Dickinson), an alluring young stagecoach passenger, and a wagon train carrying fuel and dynamite operated by Chance's friend, Pat Wheeler (Ward Bond), and guarded by Colorado (Ricky Nelson), a quiet and skilled young gunfighter.
The conflict escalates when Pat Wheeler becomes the first victim of Burdette's gunmen, shot in the back. Chance and Dude pursue the murderer to a saloon, where they disarm eight of Burdette's men, and Dude shoots the killer just as he is about to open fire on them. Subsequently, Dude is captured, leading to Nathan Burdette proposing a trade: Dude for Joe. Chance accepts the offer, but during the exchange, Dude takes down Joe with a flying tackle, sparking a fierce brawl. Stumpy becomes an unexpected savior by setting fire to one of Wheeler’s wagons filled with dynamite. The powerful explosion swiftly concludes the battle, and Chance, victorious, reunites with Feathers for good.