Vermeylen is a wealthy Dutch art collector, who lives in Amsterdam with his pretty daughter Dortje. Three crooks conspire to steal one of his paintings. The crooks are played by German actors but are apparently meant to be American gangsters (Morton and Kelly) with a French toff (DuFour) as their leader. Kelly is played by the great German actor Fritz Rasp, who must have felt he was slumming in this movie. Into this umlaut-fest arrives Niels Korff, one of those implausible fictional characters who could never exist in real life. Korff is a hugely successful author of detective novels who amuses himself as a flautist, but who acquires material for his novels by working (in real life) as an amateur detective! Korff takes it upon himself to save the Vermeylens and their painting. He is aided in this endeavour by two bumbling private detectives: van Gaalen and Schimmelpennick. —IMDb Review