One night in Hollywood, Joe Powell, the publicity director of Motion Picture Associates Studio, visits his girl friend, reporter Susan Cooper, at her apartment, and arrives during a blackout. When they hear a woman's scream, they go across the hall and find neighbor Ellen Smith strangled to death in her apartment. The building's switchboard operator tells policeman Hoffman that Ellen tried to phone the police but never finished her call. Later, Joe and Susan go out looking for studio star Mona Harrison, who has recently disappeared. When Mona's disappearance makes newspaper headlines, Joe and Susan meet with producer Hart, who is worried about the large investment he made in the film that Mona was working on before her disappearance. Police chief Williams calls a meeting at Mona's of all the people who had been at her home the night before for a dinner party. After Williams interviews maids Alice and Louise, who had quit just before dinner following an argument with Mona, and chauffeur Arthur, who has an alibi, Hart recalls events of the evening: Hart arrives at Mona's with Susan, Joe and an actress named Carol. Mona is anxious to introduce him to Ellen Smith, an ingenue, but is rude to Carol and Susan. As the servants have quit, Mona offers to take everyone to dinner at the Mocambo, and arranges to meet them there. However, neither she nor Ellen show up. After Hart concludes his story, Williams dismisses everyone, but Susan stays behind to search Mona's bedroom and finds a safe behind her vanity. Susan hides in the closet when a man enters and takes papers from the safe, then after he leaves, she is nearly strangled to death by an unseen assailant, who escapes when Hoffman enters. Hoffman arrests Susan for breaking and entering, and Hart secretly arranges to shoot the final scenes of the film out of town. To secure her release from jail, Susan tells Williams and Hoffman that Mona's disappearance is a publicity stunt and gets Joe to post her bail. Shortly after, Joe and Susan meet Hart at his office, and learn that Mona has been found strangled to death in Topanga Canyon. Joe and Susan's curiosity is piqued when unidentified film reels are delivered to Hart, and he locks them in his office closet. Later, the coroner brings in a crowd of Hollywood personalities to identify Mona's body. The police determine that although Ellen was murdered by a man, Mona was killed by a woman. Joe and Susan sneak into Hart's office and screen the secret reels, which show footage of Mona, alive, in the final scenes of her film. The usually combative Joe and Susan decide to work together, and remember that Mona had a twin sister. Joe is nearly strangled to death by an unseen assailant as he checks his office files for the sister's name. Susan grows impatient waiting for Joe and leaves, but Joe revives and tells Hart that he has seen the film. Hart admits that he found Mona in Reno, Nevada, and Joe realizes that Susan must be headed there. Concerned about Susan's safety, Hart, Carol and Joe head for Reno, and are followed by Hoffman and Williams. Susan, meanwhile, is nearly driven off the road by Arthur, who then crashes. Before dying, he tells Susan the whole story. She then goes to the hotel and when Joe, Carol and Hart arrive, she presents a signed confession from Arthur stating that he murdered Ellen and Mona's twin sister Marie. Susan, however, insists that it was Mona who has been murdered and proves that the woman claiming to be Mona is really the psychopathic Marie, after which Marie becomes violent. In her fury, Marie accidentally falls to her death from the hotel window, and Williams and Hoffman agree that Susan has once again solved a case. Susan reveals she used a fake confession to force Marie into exposing herself. Hart admits that Marie had called him from the railroad station after Mona's disappearance and asked him to take some letters from the safe, in exchange for which she agreed to appear in the final scenes of the film. Susan then calls the story in to her editor: Arthur was married to Marie for years and they were blackmailing Mona. When she refused to continue paying, Marie choked her to death and Arthur killed Ellen, a witness to Mona's murder, to protect Marie. Joe and Susan return to Hollywood just as another blackout is announced. Susan resists Joe's marriage proposal until she learns that he has enlisted in the Army.