In the town of Rio Bravo, Texas, sheriff's deputy Dude (Dean Martin), who has acquired the contemptuous nickname Borrachónnote , enters a saloon but cannot afford a drink. Joe Burdette (Claude Akins), brother of rancher Nathan Burdette, tosses a silver dollar into a spittoon. Presidio County, Texas, Sheriff John T. Chance (John Wayne) appears and kicks the spittoon away, looking at Dude with disgust. Dude is shamed by his plight and takes out his anger on Chance, knocking him out with an ax handle. Joe begins punching Dude, then shoots and kills an unarmed bystander (Bing Russell) who tries to intervene.
Joe heads to his brother's saloon, where a bloody Chance comes to arrest him for the murder of the bystander. Another patron draws his gun on Chance, but Dude shoots the gun out of the man's hand. Joe is locked up in the local jail. Chance is willing to deputize Dude, provided he can stay sober. Both remember how good with a gun Dude used to be.
Chance's friend Pat Wheeler (Ward Bond) and his wagon train of supplies stop in town, with a young gunslinger, Colorado Ryan (Ricky Nelson), riding guard. Inside the jail, Stumpy (Walter Brennan), Chance's game-legged deputy, keeps watch over the jail and Joe, who knows that Stumpy holds an old grudge against Joe's wealthy and powerful brother. Joe warns his jailers that Nathan Burdette will not like how his brother is being treated.
A mysterious woman nicknamed Feathers (Angie Dickinson) is in the saloon, playing poker. In the meantime, Dude and Chance patrol the town. Hotel owner Carlos (Pedro Gonzalez-Gonzalez) stops the sheriff, saying Wheeler has been talking too much about Chance needing help. In the saloon, Chance implores Wheeler to stop, as it will draw attention from the wrong people. Wheeler suggests that Colorado could be of assistance, but Colorado politely declines, saying he wants to "mind my own business". Colorado also promises not to start any trouble without telling the sheriff first.
Feathers leaves the poker game a winner. Chance follows her up to her room and confronts her as a card cheat, with his evidence three missing aces from the deck of cards being used in the game and a handbill indicating she is wanted for card cheating. Colorado intervenes, saying another participant in the game is the real cheat. They confront the card cheat and prove his guilt. Chance refuses to apologize to Feathers for doing his job, but is no longer in a hurry to make her leave town.
As Wheeler is walking back to the hotel, he is shot dead by a Burdette man hiding in the stable. Colorado offers to help, but is angrily turned away by Chance, who says, "You had a chance to get in this and you didn't want it." Chance and Dude flush out the shooter, who escapes into Nathan's saloon after Dude wounds him. Dude believes the man has muddy boots, but everyone in the bar has clean boots. Two of Burdette's men suggest Dude needs a drink and one throws a silver dollar into a spittoon. The bartender puts a beer on the bar, but Dude notices blood dripping into the glass from above. He turns, fires, and kills the shooter.
Chance goes back to the hotel to sleep. Without his knowledge, Feathers stands guard at the door to keep him safe, then returns to her room when he awakens. Chance discovers what she did and advises that she should leave on the next stagecoach.
Nathan Burdette (John Russell) arrives in town with his men, intent on seeing his brother Joe. Dude is standing guard and confiscating all guns. One of Burdette's men ignores him until Dude cuts one of his horse's reins with a single shot. Nathan agrees to turn in their guns until they leave.
Carlos says Feathers will not get on the stagecoach. She tells the sheriff she does not want to leave, then gives him a kiss. He indicates if he were not in such a fix at the jail, things between them might be different.
Colorado visits the jail to tell Chance the meaning of a song, "Degüello", or "The Cutthroat Song", that Nathan is paying men to play at his saloon. The song, reported to have been played by Antonio López de Santa Anna's men at the siege of the Alamo, indicates that "no quarter" will be given to one's enemy, no mercy. Dude is trying hard to stay sober. He is given back his guns (the ones he had before he left town, sold by Dude, but bought back by Chance), as well as some clothes he left behind.
The next morning, while Dude is standing guard at the town entryway, four Burdette men attack him from behind and tie him up in a stable. Others get the drop on Chance, whose rifle is just out of reach. From inside the hotel, acting on Colorado's instructions, Feathers throws a flower pot through a window a moment after Colorado steps out on the porch, distracting the Burdette men. Colorado quickly throws Chance's rifle to him and the two men shoot the three Burdette hands.
Chance decides to have his men hole up in the jail, as it will take several more days for the United States Marshal to arrive to take Joe to the Presidio. Dude's nerves are shot, but as he is about to take a drink, the sound of "The Cutthroat Song" played by Burdette's musicians steels his resolve. He and Chance go to the hotel to round up additional supplies, but Carlos and Consuelo are captured by Burdette's men, who trick Chance into charging and falling over a rope tied at the bottom of the stairs. Dude and Feathers are unable to help.
Chance is ordered to take the men to the jail to let Joe go. He is reluctant to do so, but Dude implores Chance to cooperate, saying that Stumpy is alone and has no food or water to hold out very long. The remaining Burdette men at the hotel take Dude hostage and Nathan Burdette offers to trade him later for Joe.
Stumpy opens fire at the jail, holding off Burdette's men. Chance and Colorado then take Joe to make the trade for Dude at a warehouse, leaving Stumpy behind because of his bad leg. During the trade, walking in opposite directions, Dude tackles Joe and they scuffle while a gunfight erupts. Stumpy and Carlos turn up and help even the odds. Dude overpowers Joe. The lawmen throw dynamite sticks at the warehouse where Burdette and his men are holed up, shooting the sticks to detonate them. After a few explosions rock the warehouse, the criminals surrender.
All is quiet in town as Chance gets reacquainted with Feathers, who models a skimpy new costume she will be wearing in her new job singing at the hotel. Chance does not approve of anyone seeing her in that outfit unless it is him, implying he and Feathers will soon be wed. After a pair of sheer tights comes floating from a window to the street, a delighted Stumpy retrieves them, but Dude cautions him to mind his own business.