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After a separation of 18 years, Edith Philips meets her twin, Margaret de Lorca, at the funeral of the latter's husband, whom Edith also loved. When Edith learns that Margaret had tricked the man into marriage, she becomes so filled with hatred that she lures Margaret to her apartment, signs her own name to a suicide note, and then shoots her sister. After changing clothes with the corpse, she moves into the de Lorca mansion and begins living her sister's life. The masquerade works until she meets Tony Collins, Margaret's secret lover. When he learns of the deception and threatens to blackmail her, Edith realizes that he and Margaret conspired to murder de Lorca. A fight ensues, and Tony is killed by the family's Great Dane. The police become suspicious and exhume the body of the dead husband. Arsenic is found, and Edith is arrested for murder. Although she tries to convince her former suitor, Sgt. Jim Hobbson, that she is really Edith, he refuses to believe her story, and she is sentenced to the gas chamber.