Hannes is supposed to play a song on an instrument at school, but he only has his grass pipe and no money for another instrument. In Bumfidel's shop, he and his friends discover an old trumpet that once belonged to a Mexican master trumpet player. With this trumpet he could certainly impress Birkenpappel, but of course it is too expensive. As fate would have it, a mysterious treasure map from the trumpet falls directly in front of Hannes' feet.