"Blackie" Norton (Clark Gable), a gambling hall tycoon in the notorious Barbary Coast, hires a promising but impoverished classically-trained singer from Colorado, Mary Blake (Jeanette MacDonald). She becomes a star attraction, and a romance develops between them. Complications arise when she is offered an opportunity to sing in the opera. Meanwhile, Blackie's childhood friend, Roman Catholic Father Tim Mullen (Spencer Tracy), keeps trying to reform him, while the other nightclub owners attempt to convince Norton to run for the City and County of San Francisco Board of Supervisors in order to protect their crooked interests.
Despite Father Tim's best efforts, Blackie remains a jaunty Barbary Coast atheist until the famous 1906 earthquake devastates the city. He "finds God" upon discovering that Mary survived.