The story is nominally about a very young boy called Coin, who is the eighth son of an eighth son - and thus a "sourceror" in Discworld's mythology, a person who is not only a magician but also a source of magic. Massive, world-destroying magic. But the story is more about Rincewind, Pratchett's favourite failed magician, and Conina (the petite daughter of Cohen the Barbarian, who wants to be a hairdresser but is far, far too good at killing people) and various other characters who are trying to prevent the end of the world.
I didn't much like the idea of the book, and Coin is essentially a non-entity - not a great thing for a person who's the main plot driver. Having just read Discworld books 2, 3, and 4 (with this being #5), I'm getting a little tired of him threatening the entire world every time to generate a story. Conina is a pretty good character, but Pratchett is bending Rincewind into occasional acts of bravery: he tries to explain it away (Rincewind has always been a horrible coward), but not terribly successfully. But I shouldn't be worrying about the plot - it's not as important with Pratchett's work as the other question, "is it funny?" And I think this one falls down fairly badly on that as well. Not his best.