Government Agent Conway, posing as a crew member of a ship, is investigating the flood of counterfeit money that seemingly is originating for a gambling ship, moored off-shore beyond the three-mile limit and operated by gangster Dave Reeves.
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.
Eddie Kenner is given a special assignment by the Army to get the inside story on Sandy Dawson, a former GI who has formed a gang of fellow servicemen and Japanese locals.
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.