This story is a sequel to The Wizzard and the Pony
The fate of the Disc, and, in fact, an entire swath of the Universe containing it, depends on the actions of Star Swirl the Bearded. Unfortunately, he left the Disc some weeks ago, and now the repercussions of that departure are being felt.
Rincewind, reluctant as ever, at the behest of the Disc's only known female wizard, must find a way to follow Star Swirl the Bearded and get him back, before the Disc is not only destroyed, but is made to have never existed in the first place.
No pressure, then.