Miami cop Bart Scott tracks down, in Cuba, a fugitive witness who can shed light in a double homicide and about the activities of a Miami mob lawyer who uses murder and blackmail in order to force the legalization of gambling in Florida.
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.
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.