Mr Moto encounters mysterious goings-on on a ship bound for Shanghai. He recognises his steward as the murderer of a man in San Francisco, and catches him trying to steal an important letter from the stateroom of another passenger, Robert Hitchings. Hitchings, son of the owner of the shipping line, falls in love with Gloria, who refuses to tell him anything about her life and disappears when they arrive in Shanghai.
In Shanghai, Mr Moto uncovers the secret which links the murder in San Francisco, the mysterious letter, and Gloria.