In a bush of the Praterauen you can find the body of a 12-year-old girl who had not come home after attending a piano matinee. A little later, little Christl Strobl, who had just won a prize at a piano competition, disappears. Moser and Stockinger find out that both girls had the same piano teacher. It turns out that the piano teacher Hoffmann has no alibi. When the two find Christl's body in the multi-storey basement of his house, the case seems solved: Hoffmann is arrested. To Moser's surprise, however, he suddenly presents an alibi. The search for the real culprit leads back into the underground cellars of Hoffmann's house...