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The United States government unofficially asks detective Philo Vance to investigate Archer Coe, an airplane manufacturer suspected of espionage. In Vienna, Vance steals the plans for the Coe bomber and escapes from Austria, but loses the plans on the way. On his return to the United States, Vance admits that he has no solid proof of Coe's double-dealing, but says that he intends to pursue the case. Together with Philip Wrede, one of Coe's employees, Vance calls on Coe, only to discover that he is dead from an apparent gunshot wound. Although the room in which he was killed was locked from the inside, making the death appear to be a suicide, Vance suspects that Coe was actually murdered. Coe's niece, Hilda Lake, admits that he had always accused her of stealing his plans, and adds that everyone who knew him had reason to kill him, including his brother, Brisbane Coe. Wrede is in love with Hilda, but she has become engaged to Tom MacDonald. When Dr. Doremus examines Coe's body, he finds that Coe died from a knife wound rather than a gunshot wound. A short time later, Vance finds Brisbane's body in the closet and suggests that Brisbane killed Coe. The neighbor's dog wanders into the house suffering from a head wound. Vance questions the neighbor, Doris Delafield, and learns that she planned to leave the country with Eduardo Grassi, an Italian airplane designer, making them suspects. Ling Toy, one of the servants, is discovered to be an agent for the Japanese government. Then Tom is stabbed. Vance solves the mystery by explaining that the killer came through the kitchen door to the library, started an argument and struck Coe on the head. He then hit the dog who attacked him in defense of Coe. Once the dog was taken care of, the killer stabbed Coe with a dagger. Coe regained consciousness and went to the bedroom, not knowing that he had been stabbed. Later, Brisbane came home and shot Coe, making it look like suicide, and locked the door from the outside, using a trick. The killer saw Brisbane, and thinking it was Coe, killed him as well. Vance lets the injured dog loose and he attacks Wrede, revealing him to be the killer, who was trying to steal the airplane plans for a foreign government.