But.when begins my ditty, Almost five hundreds years ago,To see the townsfolk suffer so From vermin,was a pitty explain

This is the introduction oe exposition to the poem. The speaker introduces the lovely river town of Hamelin in Brunswick and tells of its serious vermin problem 500 years before. Rats had overrun the city, to the point that the public demanded of the Mayor and "our Corporation" that the rats be destroyed or else the people would remove them from power.