Hoping to unmask the psychotic jewel thief known as Metaxa, detective Mr. Moto follows archaeologist Howard Stevens, who has just discovered the crown of the Queen of Sheba, on his voyage home from Egypt to San Francisco. Moto's cover as a Japanese tourist is blown when he meets his old friend, amateur detective Archie Featherstone, whom he introduces to the Stevens and fellow passenger, Eleanor Kirke. Meanwhile, in San Francisco, jewel thieves Joe Rubla and David Perez make plans to steal the crown, but decide that they must also eliminate Moto. Upon docking in San Francisco, Stevens is met by Hendrik Manderson, the philanthropist who has financed the expedition, and Fremont museum curator Professor Hildebrand and his secretary, Susan French. As Wendling, a member of Perez' gang, follows Moto in a cab, Joe tries to steal the armored car in which the jewels are housed, but he is foiled by the police and Archie, then escapes. The next day, the crowd reconvenes at the museum for the unveiling of the crown. Archie, who has witnessed the armored car robbery, is trailed by Perez, who deliberately breaks Archie's camera in order to lure him into a death trap later that night. Back at his hotel, Moto receives an urgent call from Manderson's servant Wong, who asks the detective to meet him at a Chinatown restaurant called the Laughing Buddha. When Moto arrives at the restaurant, however, he discovers that Wong has been murdered. Archie, who is also in Chinatown, tells Moto of his plan to meet Perez later that night, but Moto, recognizing the meeting as a set-up, convinces him not to go. Instead, the pair drive to the museum, where they find Hildebrand and Manderson playing chess and the phone and alarm dead. Paul Borodoff, the insurance company investigator, then appears and tries to steal the crown for himself, but he is stopped by Mr. Moto who announces that Metaxa is in the room. Just after Moto accuses Hildebrand of being Metaxa, Rubla tries to steal the jewels, but he is arrested by the police. Moto then unmasks Manderson as the real Metaxa when he attempts to steal the crown.