Frank Warren is a treasury agent assigned to put an end to the activities of a powerful mob crime boss. Frank works undercover, posing as a criminal to seek information, but is frustrated when all he finds are terrified witnesses and corrupt police officers.

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.