A secret brotherhood summons a dragon from its native dimension, so as to cause destruction and mayhem across the city of Ankh-Morpork.
Learning that a hero with a million-to-one chance can stop it, Rincewind searches for the right components to be that hero, journeying across the city, the Disc, and even over the edge, to find the necessary items, including a sword that goes "ting", a birthmark, and a magic spell.