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At the police station's detective bureau, Captain Frank Flynn tells a young man eager for promotion how tough a real assignment can be by relating the story of the Arnold case. Inspector Trent is summoned to the Forest Lake home of Edward Arnold, whose grandfather was murdered long ago. Family superstition holds that the current head of the household will die in the same way. While the window is blown open by the stormy night, Arnold suddenly dies. Arnold's physician, Doctor David Marsh, claims that his death was due to heart failure, but Trent suspects poison, and an autopsy confirms his hunch. Trent learns that Arnold had returned to his ancestral home only a year earlier, after spending thirty-five years in China, where he met his secretary and friend, John Fry. Under questioning by Trent, Arnold's ward, Janet Holt, reveals that she and David are engaged but have not yet married because Arnold disliked David. When Arnold's lawyer, Howard B. Smith, sneaks into Arnold's room, Mavis Fry, John's wife, discovers him trying to steal a woman's diary from the safe. However, she then helps Smith to escape and pretends not to know the identity of the burglar. After receiving a note from Shanghai, Trent goes to Fry and tells him he knows that he is really Arnold. In an attempt to defeat the family superstition, Arnold exchanged places with Fry. While still pretending to be Fry, Arnold tells Janet that the bulk of the Arnold estate has been left to her, and objections to her marriage have been withdrawn. Later, Trent finds the university educated houseboy, Kono, in Arnold's room, but a knife thrown through the window kills Kono before he can talk. Mavis then goes to Smith and demands that he turn over the diary, but before she can get away, it is taken by a thug who is actually a policeman in disguise. Noticing spilled ink on Arnold's desk, Trent realizes that Fry was killed when he refilled a pen that had poison placed on its tip. Arnold confesses to killing Fry and Kono, and planning to kill Smith to prevent Fry from blackmailing Janet for her estate. Rather than expose her during a trial, Arnold commits suicide with a gun Trent has left in his desk. As Captain Flynn explains to his prospective recruiter, Arnold was in truth Janet's father, but he never married her mother, and had hoped to conceal her illegitimate birth.