Charlie Chan in the Secret Service

When George Melton, an inventor developing a secret torpedo to demolish German U-Boats, mysteriously dies while opening his closet door and his secret plans disappear, the U.S. Secret Service summons detective Charlie Chan to investigate. Chan, who is visiting Washington, D.C. with his daughter Iris and son Tommy, instructs his offspring to wait for him at the hotel while he proceeds to the Melton house, but the two amateur sleuths decide to conduct their own investigation and follow him there. At the house, Chan is greeted by Secret Service agents Jones and Lewis and a roomful of people whom Melton had invited to a cocktail party prior to his death. Chan begins to question the assembled group: Mrs. Winters, a frivolous socialite; her chauffeur, Birmingham Brown; Louis Vega, a war refugee now involved in the importation of precious metals; Inez Aranto and her wheelchair-bound brother Paul; David Blake, a pompous politician; Mrs. Hobbs, Melton's housekeeper; and Peter Lasker, Vega's valet. When Vega discovers a set of forged plans hidden in the right side of the bookcase, Chan deduces that it was unlikely that Melton secreted them there because he was left-handed. Afterward, Chan sends Lewis back to headquarters to retrieve the completed coroner's report. As Chan and Mrs. Hobbs talk in a room upstairs, a mysterious figure enters the darkened hallway below and removes a painting, revealing a safe hidden behind it. Unable to open the safe, the figure disappears into the darkness. Later, Chan notices that the painting has been moved and discovers the safe. After Chan retires outside to speak to Jones in confidence, Birmingham steps up to the bar to pour himself a drink and notices the reflection of a gun trained at Chan. Birgmingham screams, causing the assailant to miss his target and flee. Chan then requests the key to the safe from Mrs. Hobbs, but soon discovers that the lock has been stripped and won't open. Proceeding to search Melton's lab, Chan, Tommy and Birmingham discover a book about electromagnetic fields written by a scientist named Von Vegan. When a shadowy assailant switches off the lights in the lab and begins to shoot at Chan and the others, Tommy hurls an explosive charge at their attacker, sending him scurrying. As Chan re-adjourns everyone in the living room, Lewis returns with the cororner's report. After scrutinizing the report, Chan calls the group into the hallway to demonstrate how Melton was murdered. Stepping into the closet, Chan pulls the light cord and explains that when Melton pulled the cord, the killer sent a lethal dose of electricity through the light switch in the living room, causing Melton to be electrocuted. Reconvening the group in the living room, Chan accuses Vega of being the celebrated electrical engineer and author Von Vegan, whose book he found in the laboratory. Chan continues that Vega, a foreign agent, killed Melton in order to steal the scientist's plans. As Vega begins to reply, he collapses, shot in the back. After whispering something to Lewis, Chan begins to question everybody about their acquaintance with Vega. When Lewis, following Chan's instructions, accuses Inez of Vega's murder and begins to manhandle her, Paul springs from his wheelchair to defend his sister. Paul then admits that after recovering from injuries he suffered in an accident, he remained in his wheelchair to test the intentions of his political enemies. Hoping to find the secret plans, Chan reassembles the group at the wall safe, and after fiddling with the lock, he opens the safe, causing a gun secured inside to fire. Chan then deduces that because Vega was shot in the back while everyone else was standing around the piano to his front, the murder weapon must be attached to the gun display on the wall behind the couch. Chan continues that the gun was fired by an electromagnetic field triggered by a switch hidden underneath the piano and accuses Peter of pulling the switch because he was afraid that Vega would implicate him in Melton's murder. After Chan dismisses everyone, Mrs. Winters hurries to leave the room but finds the door blocked by Lewis. Explaining that because she was seated at the piano, she was the only person who could reach the switch, Chan accuses her of Vega's murder and extracts the plans which she had concealed in her fur coat. After thanking Peter for acting as a decoy, Chan concludes that Mrs. Winters killed Vega because she was his accomplice in Melton's murder and feared he would confess and incriminate her.