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Detective Charlie Chan is accompanying his son Jimmy on the China Clipper from Hawaii to San Francisco, where Jimmy attends college. Also on the plane is Chan's friend, writer Paul Essex, who has just finished a mystery novel about a fake mystic. Paul is pestered by Thomas Gregory, an insurance actuary, who slyly reads a disturbing radiogram that Paul receives. When the plane lands, the passengers discover that Paul has committed suicide. Chan reads the radiogram, which warns of disaster if Zodiac obligations are ignored, then tells Paul's wife Stella about the tragedy. Gregory steals Paul's briefcase, which contains the manuscript, and while Jimmy follows Gregory, Chan goes to police headquarters. There he is greeted by Chief J. J. Kilvaine and introduced to reporter Pete Lewis and magician Rhadini. Rhadini, who operates a magic theater on Treasure Island, and Pete explain that they are on a crusade to expose fake mediums. Their primary target is Dr. Zodiac, whom they suspect is behind the suicides of three of his clients. Chan believes that Paul is the fourth such suicide and states that a suicide induced by blackmail is really murder. Chan, Pete and Rhadini go to Zodiac's mansion and ask to consult with the heavily-masked spiritualist. While Pete and Rhadini proclaim that Zodiac is a fake, Chan cautions that they must be careful because of Zodiac's obvious mental delusions. Later, Chan attends a party Rhadini hosts in honor of San Francisco's Golden Gate International Exposition, where he is fascinated by the mindreading abilities of Eve Cairo, who is Rhadini's assistant and Pete's girl friend. After the party, Chan, Jimmy, Rhadini and Pete return to Zodiac's mansion and find the information with which he has been blackmailing his clients. Chan burns the files, and the next day decides to use Zodiac's vanity to trap him. Rhadini issues a public challenge to Zodiac to subject himself to examination at his theater. Zodiac comes to the theater, but while Rhadini is levitating Eve, the man in the Zodiac mask is murdered and revealed to be Abdul, the medium's servant. Kilvaine then states that Gregory is actually Stewart Salzbury, an insurance detective investigating the suicides, and they offer to help recreate the levitating trick to determine who killed Abdul. When Rhadini performs the act again, he is stabbed in the shoulder. Chan then employs Eve's mindreading ability to help identify the culprit, and while he is questioning her, a hand with a gun appears. The gunman is revealed to be Rhadini, who, as the real Zodiac, had Abdul pose as Zodiac to fool the police into believing that he was dead. Chan demonstrates how Rhadini used his magic wand as a blowgun to kill Abdul and then wounded himself to deflect suspicion. After Rhadini is arrested, Chan smiles as Jimmy falls through a secret trapdoor and releases a pigeon.