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Late one night, following a series of arguments, wealthy John Carter is murdered in his car in a New York park. After Carter's body is found in his study by Kendall, the butler, police inspector Oscar Piper is assigned to the case. Soon, Oscar's girl friend, Hildegarde Withers, a schoolteacher and clever amateur sleuth, also joins the investigation. Oscar's first suspect is pretty Alice Stevens, who was with Carter seconds before he was murdered, and her boyfriend, Robert Wilkins, whose jealous tirade against Carter had been witnessed by a policeman. While inspecting Carter's house, however, Hildegarde discovers a mysterious jewel case containing a large emerald and learns from a jeweler that the gem is the famous Sultan emerald, which had been stolen years before from the Louvre. Under pressure from Oscar, Robert admits that, to protect Alice, he had returned Carter's corpse to his garage but not to the study. After learning from Kendall that Carter had argued with a French-accented stranger, Hildegarde tells Oscar, a reluctant listener, about the jewel. Oscar and Hildegarde then return to Carter's to question Joe, the chauffeur, and while looking at photographs of a decorative cup in the study, Hildegarde is knocked out by a masked thief, who steals a cup from a safe that resembles the one in the photographs. Sure that her jewel theory is correct, Hildegarde determines that the cup is the valuable Cellini Cup, which is housed in the city's Cosmopolitan Museum. While Hildegarde is at the museum, a guard is murdered and the cup is stolen. After an exhaustive search for both the cup and its fake, as well as rounds of questioning of suspects, Oscar and Hildegarde deduce that a gang of thieves connected to Carter, which includes Joe, a sculptress and a midget, committed the robberies and that Joe murdered Carter.