A Barbary Coast club owner falls in love with an opera singer, while his priest-friend objects to her lowering herself to his level.
During 1906 new year's celebrations Blackie Norton (Clark Gable) goes to a fire on the Barbary Coast. Mary Blake (Jeanette MacDonald) was burned out and asks for a job singing. Blackie offers her $75 a week, and she faints. Blackie boxes with Father Tim Mullin (Spencer Tracy). Men ask Blackie to run for supervisor to get fire regulations. Blackie gives them drinks as Mary sings "San Francisco." Jack Burley (Jack Holt) tells Blackie not to run. Mary wants to sing opera. Blackie tells Burley that Mary has a two-year contract with him. Blackie orders Mary to sing in Father Tim's mission, and he tells her that Blackie bought the organ while thriving in a wicked city. At a rally Blackie speaks for fire laws and challenges the Knob Hill landowners. He dances with Mary and on the way home puts his arm around her even though she's never had a man. At the Paradise Mary sings "Would You?"
Burley asks to buy Mary's contract and reminds Blackie his gambling is illegal. Blackie lets her decide, and she stays. Blackie tells Mary that he is crazy about her, but he doesn't believe in God. They kiss, and Blackie gives champagne to all his employees. Trixie is sad, and Mary leaves. Father Tim tells Blackie that Mary is singing opera for Burley, who asks her to marry. At her opening Blackie tells Burley the sheriff will close the show because of her contract; but Blackie watches her sing Marguerite in Faust and stops the sheriff by socking him. A happy Mary asks Blackie to marry, and he agrees. Mary tells Father Tim, and Blackie tells Burley. Blackie takes her back to the Paradise Cafe and exposes her legs. Mary asks Blackie to set the wedding date; but he says after the election so he won't be embarrassed. Father Tim tells Blackie not to exploit Mary, and Blackie hits him in the mouth. As Mary leaves, the Paradise is raided.
Burley takes Mary to his mother (Jessie Ralph), who warns her about Blackie. The Paradise is closed down the night of the $10,000 contest. Blackie reads that Mary is engaged to Burley. Mary sings opera. Burley takes her to the Chickens Ball. Della tells Blackie that Burley had him closed. For the Paradise Mary sings "San Francisco" and wins the contest; but Blackie rejects the trophy. The earthquake destroys buildings. Blackie gets out of the rubble and helps others. A major after-shock hits. Blackie finds Burley dead and looks for Mary. Fires start and spread, as water mains are broken. The army dynamites buildings to stop the fire. Mrs. Burley tells Blackie that they need God's help. Blackie finds Father Tim helping the injured. Father Tim takes Blackie to a camp, where Mary sings "Nearer My God to Thee." Blackie thanks God on his knees, and Mary sees him. Boys report the fire is out, and people plan to build a new San Francisco. They sing "The Battle Hymn of the Republic." The destroyed city fades into the new one.
This classic film contrasts the transcendental faith of Father Tim and Mary to the strong self-confidence of Blackie, who mocks "hocus pocus." Blackie and Mary are very attracted to each other sexually; but she is on a different cultural level, and while Blackie is impressed by opera, he isn't ready to adapt until the trauma caused by the earthquake seems to transform his skepticism into a deeper understanding and gratitude.