The plot of this Sailor Moon movie is an allusion to the popular tale of: The Pied Piper of Hamelin.
The man who played the flute used his flute to control and lure the rats to the river where they all drowned. The Townspeople had promised him to pay him, but at the end they failed at their promise. The man left angrily, but returned later seeking revenge. This time, he used his musical magic to attract the children of the town and One hundred and thirty boys and girls followed him out of the town, where they were lured into a cave and never seen again.