In Hadleyville, a small town in New Mexico Territory, Marshal Will Kane, newly married to Amy Fowler, prepares to retire. The happy couple will soon depart for a new life to raise a family and run a store in another town. However, word arrives that Frank Miller, a vicious outlaw whom Kane sent to jail, has been released and will arrive on the noon train. Miller’s gang—his younger brother Ben, Jack Colby, and Jim Pierce—await his arrival at the train station; it is clear that Miller intends to exact revenge.
For Amy, a devout Quaker and pacifist, the solution is simple—leave town before Miller arrives, but Kane’s sense of duty and honor are strong. “They’re making me run,” he tells her. “I’ve never run from anybody before.” Besides, he says, Miller and his gang will hunt him down anyway. Amy gives Kane an ultimatum: She is leaving on the noon train, with or without him. While waiting at the hotel for the train, she meets Helen Ramírez, who was once Miller’s lover, and then Kane’s, and is leaving as well. Amy understands why Helen is fleeing, but the reverse is not true: Helen tells Amy that if Kane were her man, she would not abandon him in his hour of need.
Kane’s efforts to round up a posse at the tavern, and then the church, are met with fear and hostility. Some townspeople, worried that a gunfight would damage the town’s reputation, urge Kane to avoid the confrontation entirely. Some are Miller’s friends, and resent that Kane cleaned up the town in the first place. Some are of the opinion that their tax money goes to support local law enforcement and the fight is not a posse’s responsibility.
Kane’s young deputy Harvey Pell, who is bitter that Kane did not recommend him as his successor, says he will stand with Kane only if Kane goes to the city fathers and “puts the word in” for him. Kane rejects the quid pro quo, and Pell turns in his badge. Kane visits a series of old friends and allies, but none can (or will) help: his predecessor, Marshal Howe is old and arthritic; Judge Percy Mettrick, who sentenced Miller, flees on horseback, and urges Kane to do the same; Herb Baker agrees to be deputized, but backs out when he realizes he is the only volunteer; Sam Fuller hides in his house, sending his wife to the door to tell Kane he is not home. Jimmy is a good person and genuinely offers to help Will, but he is vision impaired, drunk and likely to get himself killed; Kane sends him home for his own safety. The other offer of aid comes from a fourteen-year-old boy; Kane admires his courage but rejects it as well.
At the stables, Pell saddles a horse and tries to persuade Kane to take it and leave town. Their conversation becomes an argument, and then a fist fight. Kane finally knocks his former deputy senseless, and returns to his office to write out his will as the clock ticks toward high noon. He then goes into the street to face Miller and his gang alone. In one of the most iconic shots in film history, the crane shot widens to show Kane standing alone on a deserted street in a deserted town.
The gunfight begins. Kane guns down Ben Miller and Colby, but is wounded in the process. As the train is about to leave the station, Amy hears the gunfire, leaps off, and runs back to town. Choosing her husband’s life over her religious beliefs, she picks up the handgun hanging inside Kane’s office and shoots Pierce from behind, leaving only Frank Miller, who grabs Amy as a shield to force Kane into the open. Amy claws Miller’s face and he pushes her to the ground, giving Kane a clear shot, and he shoots Miller dead.
Kane helps his bride to her feet and they embrace. As the townspeople emerge and cluster around him, Kane throws his marshal’s star in the dirt, glares at the crowd, and departs with Amy on their wagon.