Retired gem merchant Peter J. Martin hires detective Dan McRae to guard a large diamond which he plans to present to his daughter Ruth on her twenty-first birthday. Notorious jewel thief Gentleman Joe overpowers McRae, however, and takes his place at Ruth's party.
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.
A corrupt D.A. with governatorial ambitions is annoyed by an investigative reporter's criticism of his criminal activities and decides to frame the reporter for manslaughter in order to silence him.
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.
Upon waking up to the news that the man she’d gone on a date with the previous night has been murdered, a young woman with only a faint memory of the night’s events begins to suspect that she murdered him while attempting to resist his advances.