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Beautiful composer Laura Ramsey (Elissa Landi) is the principal suspect when her lover, philandering singer Victor Legrand (Gilbert Roland), is found murdered in his apartment. Having "bugged" Legrand's room with Dictaphone equipment, detective John Bruce (Ralph Bellamy), who happens to be Laura's former husband, decides to tighten the noose around his ex-wife's neck by fabricating evidence against her. Our heroine's current husband, Paul Ramsey (Neil Hamilton), nobly confesses to the murder himself and is carted off to prison. Seeking revenge against Bruce, and hoping to ascertain the killer's identity herself, Laura turns the tables on the detective by recording a few of his intimate conversations. Woman in Room 13 is sometimes cited as the first film to use a hidden microphone and a secret recording device as a plot development, though chronologically it was preceded by the lesser-known Sin's Payday.