Beloved priest Father Thomasino is murdered in a San Francisco alley, and the police have few clues. But traffic cop Joe Martini becomes obsessed with finding the killer; he suspects Sylvio Malatesta. Ordered off the case, Joe turns in his badge and investigates alone. Soon he is a close friend of the Malatesta family, all delightful people, especially lovely cousin Anna. Uncertain whether Sylvio is guilty or innocent, Joe is now torn between old and new loyalties.

When Father Tomasino, a much-loved priest, is stabbed to death in a dark alley, the Italian-American community of North Beach in San Francisco is stunned. No one is more shocked and angry, however, than traffic officer Joe Martini, a former orphan whom Father Tomasino had guided into adulthood. Explaining that the priest was the closest thing he had to family, Joe asks homicide detective Lt. Kilrain if he may assist with the murder investigation. Kilrain impatiently sends him away, and after Joe reveals that a man named Sylvio Malatesta was enduring "the tortures of the damned" during the funeral, and should therefore be considered a suspect, the lieutenant angrily threatens to fire him. Determined to discover the truth, Joe resigns from the force and visits Sylvio at his waterfront restaurant. Joe introduces himself as a friend of Father Tomasino's and explains that the priest thought Sylvio might be able to give him a job. Sylvio treats Joe kindly and invites him home for dinner, and Joe is touched by the jocularity and affection that fills Sylvio's home. It soon becomes apparent that Sylvio, his mother, and his younger brother "Peanuts" are earnestly seeking a husband for Sylvio's pretty cousin Anna, who recently has come from Italy to live with the family. Sylvio takes a liking to Joe and convinces him to move in with his family. Joe soon finds that he, along with so many others in the neighborhood, is becoming fond of the generous and good-natured Sylvio, although because Sylvio restlessly paces the floor at night, his suspicion lingers. Anna explains that Sylvio lost the woman he loved while fighting in Europe during World War II, and that he has been tormented by this ever since. Soon Joe and Anna fall in love, and when Joe becomes convinced that Sylvio was playing cards at the Vallejo Club on the night of the murder, he forgets his suspicions and proposes to her. During their festive engagement party, Sgt. Jack Gillen, Joe's old friend from the police force, beckons Joe into his car. Gillen and Detective Frank Wilkins warn Joe that Sylvio's alibi was a lie. On the night of the murder, Sylvio accompanied his friend, Charlie Cuneo, to the Horizon Club, where Charlie had arranged to meet a married woman named Veda Pinelli for a date. Charlie and Veda then departed, leaving Sylvio alone for the evening. Later that night, Anna demands to know what is gnawing at Joe, but he remains silent. Visiting the orphanage the next day, Anna learns that Joe was once a policeman, but at the station, Gillen refuses to tell her why her fiancé left the force. Upstairs, Joe and Kilrain learn from Veda that Sylvio had been absent from the Horizon Club when Father Tomasino was killed. Joe persuades Kilrain to let him probe Sylvio for a motive before making an arrest. Returning home, Joe seems desperate but refuses to tell the tearful Anna what is wrong. He visits Sylvio at the darkened restaurant and delivers a story intended to test his friend's innocence: The police believe that Joe is the priest's killer. The man who can destroy his alibi is blackmailing him, and he plans to kill him. Sylvio warns Joe not to commit this crime, as it will eat away at him. Sylvio then confesses that because his previous sweetheart had planned to leave him, he killed her. Realizing that Sylvio must have confessed his sin to Father Tomasino, Joe finally accuses Sylvio of the priest's murder. Sylvio admits to killing Father Tomasino, whose gentle, knowing eyes tormented him, but explains that he was only trying to protect his family. Sylvio attacks Joe, but when the young man gasps that he, too, would do anything to protect his family, Sylvio throws down his knife and rushes into the street. A truck hits him, and as he lays dying, Sylvio asks for and receives Joe's forgiveness. Later, Joe tells Gillen and Kilrain that he was unable to confirm that Sylvio was the murderer, and then attempts to console his new family.