Jim Douglas (Gregory Peck) is a rancher who has been pursuing the four outlaws who he believes murdered his wife six months earlier. He rides into the town of Rio Arriba, where four men, Alfonso Parral (Lee Van Cleef), Bill Zachary (Stephen Boyd), Ed Taylor (Albert Salmi) and Lujan (Henry Silva), fitting the description of the men he seeks, are in jail awaiting execution. The town has issued instructions to only allow the hangman Simms (Joe DeRita) to enter the jail, so Douglas is taken to Sheriff Eloy Sanchez's (Herbert Rudley) office to state his business. The town has never had an execution before, so they have brought in a man from another town for do the job. The sheriff allows Douglas into the jail to see the men. They claim they had never seen him before, but he isn't persuaded.In town Douglas sees Josefa Velarde (Joan Collins), who he had met five years previously while in New Orleans. When Douglas originally met her, Josefa had been husband hunting. Since then, she has been looking after her father's ranch after his death. Douglas reveals to her that he has a daughter (Maria Garcia Fletcher). Other inhabitants of the town include businessman Gus Steimmetz, his daughter Emma (Kathleen Gallant) and her fiancé Tom (Barry Coe).Simms, the executioner, arrives the same day. He drinks with Douglas but is generally unfriendly. The sheriff invites Simms to see the men he is to hang, but Simms chooses to wait until the town's residents are at church for a pre-execution mass.While pretending to evaluate the men he is to hang, Simms stabs the sheriff in the back. The sheriff shoots and kills Simms in the struggle, but the inmates strangle the sheriff until he is unconscious.The four take his keys and escape, taking Emma as a hostage. The wounded sheriff interrupts the church service and tells the townspeople that the prisoners have escaped. The townspeople form a posse to ride out immediately and enlist Douglas to help them track down the outlaws, but Douglas is in no hurry and waits until morning. The posse pauses at a mountain pass and then they decide to leave Parral behind to prevent anyone from following them through the pass. Douglas catches up with the posse the next day at the pass, where they are held up trying to dislodge the outlaw.Douglas corners the outlaw Parral and shows him a picture of his wife in the cover of his pocket watch. Parral says he's never seen her before. Douglas accuses him of being one of the four who killed his wife. Parral denies it, pleading for his life, but Douglas kills him. Douglas follows the outlaws and next tracks down Taylor. He lassos Taylor by the feet and hangs him upside-down from a tree, leaving him to die. The two remaining men reach the house of John Butler (Gene Evans), a prospector and Douglas's neighbor. Zachary kills Butler and Lujan steals the sack of coins that Butler tried to run off with. Zachary attacks Emma, but Lujan sees someone approaching in the distance and tells Zachary they must leave, and after Zachary rapes Emma they leave. Josefa and Douglas arrive at Butler's cabin from different directions where Douglas identifies Butler's body. The posse arrives and finds an injured Emma in Butler's cabin. Douglas tells the posse to ride on while he goes back to his ranch to get fresh horses.When Douglas arrives at his home he finds that the fugitives have taken his last horses. Leaving Josefa with his daughter, he rejoins the posse. When they arrive at the Mexican border, the other members of the posse refuse to cross the border and Douglas goes on alone. He finds Zachary in a bar and shows him the photo of his wife. Zachary says he has never seen the woman before and asks what Douglas wants. Douglas insists that Zachary is one of the four who killed his wife, and Zachary pulls his weapon in self-defense, but Douglas kills him first.Douglas see Lujan run away. Lujan greets his wife and learns that his son is ill. Douglas tracks Lujan to his home and arrives to find him fetching water. Lujan's wife, seeing that Douglas intends to harm her husband, knocks him out with a clay pot. Regaining consciousness, Lujan holds Douglas at the point of his own gun. Douglas shows Lujan the photo of his wife. Like the other three, Lujan says he has never seen the woman before. Douglas asks why Lujan is riding one of Douglas' horses, and Lujan says that he and his companions rode past the Douglas ranch on their way from the border. Pointing to the sack, Douglas states that the men who killed his wife stole the sack containing the family's life savings. When Lujan tells Douglas that he took the bag from Butler's dead hand, Douglas realizes that Butler killed his wife.Douglas realizes that he has been pursuing men who had nothing to do with his wife's death. He remembers that he showed each of them a picture of his wife, and all of them denied ever having seen her. Douglas sees that he is no better than the men he has been pursuing, having murdered them in cold blood. He returns to the town where the hanging was to take place and goes to the church to beg forgiveness from the pastor. The priest (Andrew Duggan) says that Douglas did what he felt was right and to his credit, does not take refuge in the fact that they were outlaws anyway. Josefa then arrives with Douglas' daughter, and as they exit the church together, the sheriff and the townspeople celebrate Douglas' success at tracking down and killing the men.