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Johnny O'Clock, a sharp-witted New York gambling house overseer, has many "friends" in the city's underworld: His associates include Pete Marchettis, Johnny's senior partner and owner of the lavish casino he operates, and Chuck Blayden, a crooked, trigger-happy cop who is investigating other gambling houses in town. After trying to cut into the casino's profits and warning Johnny under threat of death not to interfere with his intention to become Marchettis' partner, Blayden ends his relationship with cigarette and coat check girl Harriet Hobson, then disappears. Harriet is later found dead of an apparent suicide in her apartment, and Inspector Koch begins his investigation into her death by questioning Johnny, Harriet's sister Nancy, who is smitten with Johnny, and Johnny's associate, Charlie. Blayden's body turns up in a nearby river, and when it is learned that Harriet was actually murdered by poison, Johnny and Marchettis become the prime suspects in both murders. When Marchettis discovers that his wife Nelle has given Johnny a watch with the same diamond that she gave him, he suspects that Johnny is having an affair with her and sends some gunmen to kill him. Marchettis' hit men pull up beside Johnny's car as he is driving Nancy to the airport and spray the car with bullets. Johnny and Nancy survive the attack, after which Johnny roughs up Charlie and accuses him of talking to Marchettis about Nelle's interest in him. Charlie confesses that he tipped off Marchettis, and Johnny, now certain that Marchettis has betrayed him, goes to the gambling house to dissolve their partnership. Marchettis is surprised to see Johnny alive, and although he coolly consents to the break-up of their partnership, a gunfight ensues when Johnny tries to take his half of the casino's profits. Johnny kills Marchettis, and Nelle, who is still in love with Johnny, asks him to let her go into hiding with him. Johnny refuses, and Nelle takes revenge by telling the police that her husband was killed by Johnny when Marchettis caught Johnny in the act of robbing the gambling house safe. Wounded in the gunfight, Johnny hides in the gambling house while Koch surrounds the building with police. The standoff continues until Johnny takes Koch as his hostage and uses him as a shield to make his escape. However, when Johnny sees that Nancy is waiting outside the casino for him, he realizes that she truly loves him, and he has a change of heart. After surrendering his gun to Koch, Johnny gets an indication that Nancy will wait for him, and he willingly accepts his arrest.