A violin maker is found murdered in his workshop in Barcelona. The only "witness" is a blind street musician who has heard a melody played in the workshop at the time of the crime. The trouble is he cannot identify the tune. Police chief-inspector Luna and his French colleague Revel first suspect a violin player who has money problems, then Visconti, a music teacher and finally Florencio, a violin maker in love with Luce, a young dancer. At bay, Florencio confesses his crime to his young mistress and both get away to the French border. But the policemen have an original idea: they decide to use the radio contest "Melodia misteriosa" to have the listeners identify the mysterious song...