While driving down a street in Chicago, Tony Arnelo, a crooked nightclub owner, tells his mistress, Anne Parkson, that he killed his former girl friend, the actress Claire Lorrison. Tony claims that he killed Claire in order to be with Anne, and warns Anne that he will implicate her in the murder if she goes to the police. Regretting her association with Tony, Anne thinks back to the day her husband Ted, a lawyer, introduced her to Tony, a new client: Tony is immediately smitten with Anne, and Anne, neglected by her overworked husband, allows herself to fall instantly in love with him. Though torn between her devotion to her family and her attraction to Tony, Anne eventually consents to meet Tony in secret at his apartment. After Tony hires Anne to be the interior decorator at his nightclub, a furtive affair develops. Anne, however, begins to feel guilty, and tries to repair her marriage by asking her husband to take her on a long trip. When Ted refuses to leave town, citing his heavy work load, Anne decides to pursue her affair with greater resolve. Anne's infatuation with Tony soon ends, though, when she finds Claire at Tony's apartment, and when Claire tells her that Tony mistreated her. Tony, overhearing the accusation, slaps Claire, and Anne leaves in haste. The following day, Anne is shocked by a newspaper story announcing that Claire has been murdered, and that a compact case bearing the initial "A" was found in her possession. As Anne comes out of her reverie, Tony warns her of the dire consequences she faces if she tells anyone what she knows about the murder, and insists that they continue their affair. At a stoplight, Anne jumps out of Tony's car and runs home. The next day, on Ted and Anne's twelfth wedding anniversary, Ted surprises Anne by taking her to Tony's nightclub for dinner with their friends. The dinner is interrupted by a visit from homicide detective Sam Leonard, who asks Tony about Claire. Tony coolly denies any association with the actress, but Anne faints, unable to bear the questioning and innuendoes. Later, Ted finds Anne's key to Tony's apartment and realizes that his wife has been having an affair with Tony. He also discovers that the police suspect that Anne is involved in Claire's murder when Leonard shows him Anne's compact case. When Ted confronts Tony with his knowledge of the affair and demands to know the truth about Claire's murder, Tony lies to him and tells him that Anne killed Claire. Refusing to believe Tony's story, Ted begins his own investigation into the murder and discovers that Anne could not have killed Claire because she was with him at the time of the murder. Leonard eventually arrests Tony for the murder, but when Tony tries to escape, the detective shoots him. With his dying breath, Tony confesses his guilt, and Anne is exonerated. Ted returns home to find that Anne has made an unsuccessful attempt to kill herself with an overdose of pills. Anne soon recovers, and after Ted forgives her, they embrace.
