The Pied Piper of Hamlin plot
A grandfather tells his two grandchildren the story of the village of Hamelin, which is plagued by rats. When everyone in the village doesn't like it anymore, suddenly a stranger comes and promises that he will make the village 'rat-free' in exchange for 200 gold bars. The Pied Piper does what he promised using his magic flute. When the villagers don't keep their promise and give him only 1 gold ingot, he wants revenge.