Bart Allison, a stony man bent on revenge, rides into the town of Sundown with his old friend Sam on the very day that the target of his ire, Tate Kimbrough, is to marry Lucy Summerton, the town belle. Kimbrough, the self-appointed king of Sundown, readies for his wedding while his long-time mistress, Ruby James, voices her feelings of rejection and betrayal. At the Livery Stables, Bart meets Dr. Storrow, the only man in town unafraid of Kimbrough and his flunkies, Sheriff Swede Hansen and Spanish, an unscrupulous killer. After strapping on his gun, Bart goes to the wedding, stopping along the way for a drink at the saloon. Ignoring the signs that announce all drinks are on Kimbrough, Bart tosses down some coins and orders a whiskey, but when Swede toasts Kimbrough, Bart sets his drink down. Angered, Swede tosses the coins into a spittoon, and Bart then hands him the spittoon and orders Swede to retrieve them. With tension filling the room, the crowd clears out to attend the wedding ceremony. Soon after, Morley Chase, a rancher who despises Kimbrough, rides into town with his men to witness the nuptials. As Doc escorts Ruby into the church, Bart enters and stands in the back. When the minister asks if anyone objects to the union, Bart speaks up and asks Kimbrough if he remembers Sabine Pass and a girl named Mary. As Kimbrough's men rush toward Bart, Bart warns Lucy that she will be a widow by sundown and then runs out of the church. Taking refuge in the stable with Sam, Bart barricades the doors as Swede and his thugs fire at them. Lucy, unsettled by the turn of events, throws down her bouquet and rides off in her carriage, after which Kimbrough orders Lucy's father Charles to bring her back. Directed by Kimbrough to kill Bart, Swede and his men surround the stable. When Spanish tries to climb in through a window, Sam gores his hand with a hay hook. Doc, who has returned to the stable to retrieve his medical bag, treats Spanish and then sends him back to Swede. After Spanish leaves, Doc tells Bart that Kimbrough has destroyed something he loved, the fortitude of the people of Sundown. When Doc asks who Mary is, Sam blurts out that she was Bart's wife, who was "stolen" by Kimbrough. Just as Kimbrough is berating Swede for failing to kill Bart, Charles returns and reports that Lucy will not go through with the ceremony until Bart leaves town. Kimbrough then sends Charles to the stable to tell Bart that he can ride out of town alive if he immediately leaves. When Bart rejects Kimbrough's proposal, Charles offers him money to leave, prompting Bart to accuse him of "selling" his daughter to Kimbrough. Later, when Sam asks Bart why he refrained from killing Kimbrough at the church, Bart replies that he felt Kimbrough deserved fair warning. As the wedding guests wait at the saloon, Kimbrough tries to convince Lucy to marry him. Experiencing second thoughts about the marriage, Lucy goes to the stable to talk to Bart. When Bart tells her that Kimbrough "stole" his wife, Lucy replies that no one can be stolen, and that Mary must have wanted to go with Kimbrough. After a furious Bart orders Lucy to leave, Sam tells him that Mary had been unfaithful many times before she met Kimbrough. Lashing out in anger, Bart slaps Sam and tells him to get out. Ravenously hungry, Sam walks out into the street in search of a meal, dropping his gun belt at the sheriff's orders. At a restaurant, Sam tells Doc that Mary, unable to face life mated to only one man, killed herself one week before Bart returned home from the war. Hoping to convince Bart to forget Mary and start a new life, Sam heads back to the stable but is shot in the back by Spanish before reaching his destination. With his dying words, Sam beseeches Doc to tell Bart that "Mary was no good." Outraged by Sam's murder, Doc tries to rouse the town folk from their moral torpor. When Doc, trying to rally the crowd to oppose Kimbrough, argues they must all take a stand for justice, Charles admits that Kimbrough robbed him of his self-respect. After Doc enlists Morley's help in assuring that Bart will be given a fair chance, Morley and his men disarm Swede's thugs. With the odds now evened, Bart emerges from the stable for a showdown with the sheriff. Bart outdraws Swede, but loses his balance and cuts his hand on a carriage wheel. While Doc bandages Bart's hand and pleads with him to give up his quest for vengeance, Ruby implores Kimbrough to leave town. Ignoring her pleas, Kimbrough straps on his gun belt and steps into the street. As the town watches, Bart appears, his gun hand bandaged, his gun now strapped to his left hip. As the two adversaries face off, Ruby fires her rifle, wounding Kimbrough in the shoulder, thus forestalling the gunfight. When Ruby declares that Bart "never really had a wife," and cannot revenge something that he never had, Doc agrees. Some time later, Ruby and Kimbrough, his arm in a sling, leave town for good. When Morley calls for a celebration, Bart, holding the town responsible for Sam's needless death, rides out of Sundown leading Sam's riderless horse.
