Father Keogh arrives at the Mexican town of Quantana, where he is to replace Father Gomez as the local priest. The beautiful Locha de Cortinez warns him about the bandit Anacleto, who effectively runs the town and has deliberately let the church fall into disrepair.
Gomez tells Keogh more about Anacleto, raised to hate religion because of its association with a corrupt government. When Gomez attempted to convert him, people started dying in alphabetical order: A, B, C. Keogh and Gomez walk through the town to the bus station, but they are surrounded and jeered at by Anacleto's men. Keogh cleans up the church, but when he rings the bell for Mass, only three elderly women turn up.
Keogh visits the town dignitaries, all of whom claim Anacleto isn't a problem. The police captain is more honest, but can't protect a town that doesn't want to be protected, and Anacleto always has an alibi. The wealthy Pedro de Cortinez (Locha's father) admits paying protection money for the sake of a quiet life.
Keogh's congregation grows, but Anacleto's men intimidate them. Undaunted, Keogh preaches in the town centre. Anacleto decides that Keogh needs to be disposed of, but he survives two assassination attempts. However, the original murders resume with D, E and F - the latter just a child.
Furious, Keogh goes to see Anacleto, who says that it's a pity he won't co-operate with him like his predecessor. Keogh says that he's not beyond redemption, but Anacleto laughs.
Keogh gets a tip-off that Anacleto's men will pay old Gonzalez a visit. Noting the surname, the police attempt to catch him in the act, but Anacleto was expecting them. During their conversation, the informant Francisco Goya is murdered.
Anacleto's right-hand man Old Uncle drinks too much and summons Keogh to hear his confession. Anacleto later visits Keogh and demands to know what Old Uncle told him. Old Uncle, thinking that he might have said too much, attempts to kill Keogh, but Anacleto shoots Old Uncle instead. The police arrive to arrest Anacleto, but Keogh sends them away. But this incident fatally damages Anacleto's hold on Quantana: he's revealed his vulnerability and is forced to leave.
A year passes and the church prospers. Anacleto returns, and asks Keogh to give him religious instruction. Keogh agrees to let him stay.
Locha's mother discovers love letters by her daughter, apparently addressed to Anacleto, and decides to send her away to Florida. She shows them to Keogh to attempt him to persuade her. Keogh discusses the matter with Anacleto, who denies receiving any letters. He secretly reads one, correctly believes that the object of Locha's affection is really Keogh, and gets her to confess this. Locha agrees to go to Florida. Anacleto returns to his henchmen, who are hiding out in a remote villa, and says that he's obtained useful information on Keogh.
Locha gets engaged to an American, Phil Brown. They return to Quantana for the wedding, but Locha's doubts are clear. When Anacleto drives up in front of the church, she impulsively leaps into his car - since she did this of her own free will, the police can do nothing.
Anacleto goes to confess to Keogh, revealing Locha's feelings for him and warning that she may be suicidal. He offers to take him to her, but on condition that Keogh lets Anacleto resume control of Quantana. Keogh sees Locha, but makes it clear that he cannot return her love.
In the church, Keogh's sermon initially seems to be going along the lines agreed with Anacleto, but ends with a firm denunciation. Anacleto is arrested, and Keogh says that he'll give evidence against him. Everyone is delighted, but a downcast Keogh realises that this betrayal means that he will never be able to redeem Anacleto.
The police escort is ambushed. Anacleto escapes but is fatally wounded. Keogh runs up to him, but is shot in the back by one of Anacleto's men. Lying in each other's arms, the dying Keogh tries to get Anacleto to repeat an act of contrition, but Anacleto realises that he was always more interested in the singer (Keogh himself) instead of the song (his faith).
