Hans Albers plays the detective Morris Flynn, and Heinz Rühmann his assistant Macky McMacpherson. They stop a train to Brussels at night, and due to their behaviour and clothes, personnel believe that they are Sherlock Holmes and Dr. Watson. This is indeed Flynn’s and McMacpherson’s intention, but they claim not to be the famous detective and his friend, and they urge them not to tell anybody. They interrogate people in order to make their cover perfect, among them two attractive sisters, Mary and Jane Berry, who travel to accept their uncle’s heir.
In Brussels the would-be Holmes and Watson stay at Hotel Palace, and immediately police asks them to solve a case. During the 1910 World Exposition, stamps have been stolen and replaced by fcounterfeits. Observed and later attacked by gangsters, Flynn and McMacpherson find out that the two sisters’ uncle has been the forger and boss of the gang. Their working place is found in the cellar of the uncle’s castle the sisters inherited.
Flynn and McMacpherson are put on trial for impersonating the famous detective Sherlock Holmes and his friend. Then a laughing man stands up, who had observed the duo several times, and reveals himself as Arthur Conan Doyle. He explains that Holmes and Watson are fictional characters created by himself. Flynn and McMacpherson get engaged with Mary and Jane Berry.