Before Charlie Chan can catch a pair of murderers he has to prove their victims are really dead.

Disguised as a policeman, a murderer breaks into the mansion of Harper, a scientist, kills him and disposes of his body. Charlie Chan, the world famous Chinese-American detective now working as a government agent, is called to the case by police inspector Godfrey, who informs him that the scientist was working for the government to create a formula whereby wood could be made as strong as metal. At the scientist's mansion, Chan and Godfrey question, with the help of the local authority, Sheriff Mack, the inhabitants of the Harper household: Louise Harper, his sister; Jean Kent, his orphaned niece; Roth, his butler; Michael Strong, his mute chauffeur/handyman; and Walter Meeker and Stella Graham, his assistants. Chan quickly learns that all of them hated Harper, whose cruelty went so far as to force his relatives to work for him as servants. Meanwhile, Chan's two assistants--Eddie, his fourth son, and Birmingham Brown, his chauffeur--arrive at the Harper estate, having been thrown out of their hotel room. Later, Chan discovers the murdered Harper inside the scientist's gas chamber, and Meeker tells him that all of Harper's work will be rendered useless unless Chan can find the scientist's secret formula. After the local coroner, Dr. Samuel R. Peabody, declares that Harper died of "natural causes," Chan and Mack visit Peabody, who then discovers a poisonous dart inside the dead scientist's mouth. Later, Chan learns that Jim Kimball, a missing local policeman, was having a secret romance with Jean, but she insists that he was not the officer seen in the mansion at the time of Harper's disappearance. Roth is killed soon after he promises to tell Chan the name of the murderer. After examining the dead butler's quarters, Chan tells Mack that Roth was killed in his room, despite the fact that both Eddie and Birmingham saw the butler walking downstairs shortly before his death. Lloyd Archer, Harper's step-son, then arrives at the estate, claiming that Harper's secret gas formula was stolen from his birth father. That night, Chan is nearly killed by a poisoned dart while searching for clues. The next morning, Louise discovers her brother's secret hiding place, but the vault itself can only be opened by Harper's voice speaking a coded message. Chan, realizing that someone inside is being gassed, opens the vault by playing a dictaphone recording of Harper. An unconscious Jean is then removed from the vault, but she later insists that she was not attempting to steal her uncle's formula. Michael is later killed after he writes Eddie a note, claiming to know who the murderer is. Kimball's dead body is then uncovered in the gas chamber by Birmingham, and Chan announces that all the murder victims were killed by an air gun hidden inside a ventriloquist's dummy, and that the dead Roth walked with the aid of puppeteer's strings. Knowing that Stella is a master puppeteer, Chan names her as the murderer's accomplice, then states that the life mask broken during Michael's murder was one of Meeker. The laboratory assistant attempts to escape, but is captured, after which Chan pulls a rubber mask from his face to show that he is really Archer, not Meeker, whose murdered body is later found in the basement. Chan then convinces Stella to tell all, and she states that Archer, a vaudevillian quick-change artist and impersonator, is actually her husband. With the case solved, Chan thanks his son for his assistance, though he is unsure of what assistance he was.