Nils Korff (Heinz Rühmann) is a good-natured, dachshund-owning and slightly naive flutist in a Danish spa orchestra. Initially he has no idea that a couple of Dutch private detectives have published a famous series of crime novels in his name, based on the real crimes of a gang of art thieves. The private detectives lure him to Amsterdam, where he is made to play a comical role in a variety show in the "Trocadero" theater operated by the gang. The gangsters are surprised by this and naturally consider it a provocation by the supposed snoop Korff, so they make an attempt on his life. The flutist barely survives the performance through luck and the help of the private detectives. Having fallen in love with an art collector's daughter, the flutist takes on a more active role in preventing the theft of her father's Rubens painting and in catching the gangsters.