^The rats overran this town. The townspeople accepted an offer from the Pied Piper to get rid of them. Of course, as long as they payed him afterwards. He led them into the river playing his flute/pipe. The townspeople were all happy and thought "we don't need to pay him, the rats are gone". So they didn't.

This angered the Pied Piper. To punish them he led all the towns children with his flute/pipe playing. Into the river and their deaths.

The moral is?..

..don't trust men in tights and funny hats. Maybe.