Breathless is first seen performing at her current lover, ‘Lips’’, casino. When Lips and Breathless leave, they are kidnapped by Big Boy and taken to a warehouse. There, Lips is killed by being covered in cement and Breathless becomes Big Boys ‘property.’ She accidentally leaves her earring there, which Dick Tracy finds on the floor.
She is later seen at the casino, now also owned by Big Boy attempting to perform but it is ruined by Big Boy trying to ‘improve’ it. Big Boy slams the piano lid on the pianist's fingers, and Breathless remarks that it wasn‘t very wise to do that because he’s the best pianist in town. An angry Big Boy threatens the beautiful Breathless. Tracy comes in and arrests Big Boy, and Mahoney leaves to her dressing room. Tracy follows her there, and she flirts with him unashamedly.
Breathless later goes to the police station to seduce Tracy. She lies down on the desk seductively and drinks his alcohol. Mahoney tries to kiss him, but he doesn’t kiss her back. A confused Breathless once again tries to flirt with him, but he drops the earring she left on the floor of the warehouse into the drink; she angrily leaves in a bad mood.
Breathless once again shows up in Tracy’s life, this time coming to his apartment wearing an extremely low cut top. She notices the picture of his girlfriend Tess, and quickly seduces him, kissing him. Tess catches the two in the act and the femme fatale leaves. Despite his adultery, Tess does not leave Tracy.
Later she meets Tracy at the dock and tries unsuccessfully to seduce him again, but he tells her that he loves Tess. A jealous Breathless then refuses to testify at court.
In secret, the scheming showgirl works out a way to double-cross the newly released Big Boy, and decides to do this by donning a disguise and becoming a faceless male gangster called ‘No Face.’ Using this disguise, she kills two of Big Boys henchmen when they try to kill Tracy in the same way as Lips.
Later, Breathless kidnaps Tess, and baits and incapacitates Tracy and gives him to Big Boy as part of a twisted scheme to end Big Boy. Mahoney, disguised as No Face, frames Big Boy for Tess’ kidnapping by ‘tipping off’ the police. THis causes Big Boy to turn insane and kidnap Tess for real. When Tracy storms into the casino, she tells him where he and Tess went.
In the final scene of the movie, Tess is tied to machinery and Tracy and Big Boy are fighting each other. The callous villainess, disguised again as No Face, holds fighting pair at gunpoint and tries to persuade Tracy to team up with ‘him.’ Tracy refuses and edges closer to Breathless, but she cannot shoot him because she loves him. Suddenly, ‘The kid’, a nameless street boy Dick and Tess found, tackles Breathless and Big Boy uses that chance to shoot No Face. Tracy then sends Big Boy falling to his doom.
Tess notices that the faceless gangster is wearing a mask, and they pull it off and discover the dying face of Breathless Mahoney. Dick gets Tess to call an ambulance, and he is left alone with the sultry bad girl. The unrepentant villainess asks Tracy is he loved her, he responds by kissing her. Breathless dies in his arms and Tracy marries Tess.