The ruthless Solly Caspar is fighting to retain control of Bay City's criminal activities when Frank Jansen (Kent Taylor), an honest man and mayoral hopeful, begins a strong anti-crime campaign. Caspar tasks his right-hand man Ben Grace to dig up some dirt on the candidate and ruin his chances of election.
Ben follows the candidate's redheaded secretary, June Lyons (Rhonda Fleming), to a jail where she's picking up her equally scarlet-tressed and sexy kleptomaniac sister Dorothy (Arlene Dahl). June is Jansen's girlfriend as well, but their relationship is still only social, and there's nothing to work with—but in the process of following her, Ben has become attracted to June.
Ben gives June incriminating evidence about Caspar, who slapped him around for not providing any dirt on June's boss. A tape Ben made proves Caspar killed a crusading newspaperman supporting Jansen, and Caspar is forced to leave the city. Ben takes over the rackets, unbeknownst to June.
Meanwhile, her sexually charged sister is attracted to Ben. She makes a play for him at a beach house previously belonging to Caspar and nearly kills Ben by accident with a spear gun. She goes for a swim in a leopard-pattern bathing suit, and afterwards we see them on the sofa, her fully dressed and looking very satisfied, while Ben looks guilty. Learning that he's never taken June there, Dorothy says "Score one for little sister", and later tells June they had sex. June later confronts Ben about this, and he never responds directly to the accusation, but says it's June he really wants. She wonders if it's really both of them he's after.
While Ben and Dorothy are still at the beach house, one of Caspar's men who was jailed because of Ben's information comes after him with a gun, but Ben wounds him—then tells him he's the one who bailed him out.
When the police arrest Dorothy for stealing a necklace, Ben intervenes on her behalf, making June finally realize that he's not as honest as he seemed. Jansen, who loves June, insists that her sister must go back to jail.
Caspar returns for revenge and finds Dorothy alone in the beach house. She throws herself at Caspar and his money, not even minding when June shows up and Caspar decides to murder her. June shoots him with the spear gun, then twice with his own gun.
Ben arrives and wants June to go away with him and the money. She refuses. Caspar, not yet dead, wounds Ben, then gathers his men, and comes back to finish the job. Ben gives himself up on condition June and Dorothy won't be hurt, and mocks Caspar, who shoots him several times—not realizing Ben has called his friend Dietz, the chief of police, and he is just arriving on the scene with a full squad. The police enter the house and arrest Caspar and his men, breaking the gang once and for all. A badly wounded Ben is taken to the hospital, his fate uncertain, and June goes with him, leaving Dorothy with Jansen.
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