Although the youth, William Bonney, an infamous gunslinger known as Billy the Kid, maintains that he bought Red from a stranger, Doc insists that he stole him.
Shortly after he disembarks at the Lincoln, New Mexico train depot, gun-slinging gambler Doc Holliday reunites with his old friend and former partner-in-crime, Pat Garrett. When Pat, who is now the town’s sheriff, hears that the penniless Doc is searching for his horse Red, who has been stolen, he directs his friend to the local dentist’s office. There Doc finds Red tied up outside and confronts the young man who comes to claim the horse. Although the youth, William Bonney, an infamous gunslinger known as Billy the Kid, maintains that he bought Red from a stranger, Doc insists that he stole him. Despite their disagreement, Doc takes Billy’s side when Pat tries to arrest him for theft. Annoyed, Pat orders Doc and the smooth-talking, quick-fisted Billy to leave town by sundown. Doc and Billy, however, ignore Pat’s command and play cards together in the cantina. Doc wins hand after hand from Billy and is unruffled when his rival accuses him of cheating. After Doc announces he is giving Red to Billy as a gift, however, he tries to sneak the horse out of the barn, but is caught in the act by a watchful Billy. Moments later, as Billy is about to settle down next to Red, he is shot at by an unseen assailant. In the darkness of the barn, Billy overpowers his attacker, who turns out to be a beautiful woman named Rio McDonald. While struggling to free herself, Rio condemns Billy for murdering her brother and vows to kill him. Billy admits to shooting Rio’s brother in a fight over a woman, but insists that the match was fair and gives Rio a passionate kiss. The next morning, in the cantina, Billy is approached by a stranger who identifies himself only as an enemy of Pat. The stranger enlists Billy’s help in confronting Pat and suggests they stage a mock fight as practice. Suddenly sensing a set-up, Billy draws his guns one second before the stranger does and kills him. After they learn that the stranger was actually a friend of Pat, Doc advises Billy to flee, but Billy insists on facing the sheriff.
Maintaining that he was pushed into the gunfight, Billy refuses to give himself up to Pat and his deputies, and is shot by Pat. Before Pat can fire his rifle again, Doc shoots the gun out of his hand and downs two of his deputies. As Doc and a wounded Billy are about to leave the cantina, Pat angrily declares his friendship with Doc “finished.” Doc takes Billy to recuperate at Rio’s house, unaware that Rio, his girl friend, had previously tried to kill Billy. After Doc leaves, Rio contemplates stabbing the unconscious Billy, but is unable to do the deed. Instead, Rio and her aunt Guadalupe nurse Billy through fever and chills until, one month later, he is recovered. Rio admits to Billy that she is Doc’s “girl,” but gives in to his seductive charm and kisses him. Soon after, Doc returns and learns that Rio married Billy during one of his delirious periods but has not told him about their new relationship. Although Doc is angered by Rio’s change of heart, he is more infuriated by Billy’s continued insistence that Red is his horse. To resolve the matter, Billy offers Doc a choice between Red and Rio. Doc quickly picks Red over Rio, and the two men ride off toward the desert together. When they see Pat approaching in the distance, they deduce that Rio revealed their route and then discover that she filled their canteens with sand. After a thirsty night, Doc wakes to find Billy gone and Pat at his side. Pat arrests Doc, while Billy sneaks into Rio’s house and takes her by surprise. Later, on the trail, Doc and Pat find Rio tied between two rocks, abandoned with no water. Confident that Billy will return to free Rio, Doc and Pat lie in wait for him. As predicted, Billy shows up the next morning and is apprehended. While Doc and Billy argue about whether Billy is in love with Rio, a hostile Indian group sends smoke signals announcing the white men’s presence. Pat, Billy and Doc jump on their horses and head for nearby Fort Sumner, but are soon overtaken by the Indians. Reluctantly, Pat gives Billy and Doc guns, and the three men charge madly for the fort, chased closely by the Indians. By dragging cacti behind them, the men create a moving dust storm, which causes the Indians to give up their pursuit. While the men stop at a house to rest, Pat gives owner Pablo a note to deliver to the Fort Sumner marshal. Overhearing Pat and Pablo’s conversation, Doc is about to flee on Red when he is stopped by Billy, who insists once more that the horse belongs to him. Billy challenges Doc to a duel over Red, and although Billy outdraws Doc, he is unable to shoot him. Annoyed by Billy’s sudden passivity, Doc shoots the youth’s hand and then his earlobes. Stating that Doc is the only partner he has ever had, Billy refuses to fight back, however, and the two men finally reconcile. Humiliated by Doc’s obvious preference for Billy, Pat explodes with anger and shoots Doc. The next morning, after a remorseful Pat and Billy bury their friend, Pat allows Billy to leave. As he is about to ride off, Billy invites Rio to join him, and Rio happily accepts.