When infamous jewel thief Flambeau (Paul Lukas) announces his intention to steal stones from a diamond cross in Father Brown (Walter Connolly)'s church, the crime-solving cleric fights to retain the cross, and also to save the soul of the elusive Flambeau.
Childhood chums Rocky Sullivan and Jerry Connelly grow up on opposite sides of the fence: Rocky matures into a prominent gangster, while Jerry becomes a priest, tending to the needs of his old tenement neighborhood.
Recently paroled from prison, legendary burglar "Doc" Riedenschneider, with funding from Alonzo Emmerich, a crooked lawyer, gathers a small group of veteran criminals together in the Midwest for a big jewel heist.
When an employee at an illegal gambling den dies suspiciously, her sister, Nancy, looks into the situation and falls for Johnny O'Clock, a suave partner in the underground casino. Selfish and non-committal by nature, Johnny slowly begins to return Nancy's affection and decides to run away with her, but conflict within his business threatens their plans. As Johnny tries to distance himself from the casino, his shady past comes back to haunt him.
Police Lt. Leonard Diamond vies to bring a clever, well connected, and sadistic gangster to justice all the while obsessing over the gangster's girlfriend.