A full-planet reality reset on the Disc causes major chaos, as magic makes sure a failed magician with an important spell stays alive long enough to use it.
This one's an epic quest kind of novel: the guys have a mission (even if they don't know it), and they meet characters and fight battles on their way to the final goal. And Luggage shows what he's really made of!
I'm just sort of getting into the Discworld way of thinking - I've decided to read my way through them in the order they were written. They're funny and smart, with a deep philosophy that bears lingering analysis.