What is the main conflict in Sourcery by Terry Pratchett?

Sourcery by Terry Pratchett is the story of the wizards that live on Discworld and the struggle that ensues when a new sourcerer shows up. Rincewind, a most inept wizard and coward, will travel the world and face unimaginable odds, against his wishes, in order to save the dirty streets of Ankh-Morpork.