A terrible thing has happened. Now it's become clear why wizards should remain celibate. One wilful wizard, Ipslore the Red, in defiance of tradition, marries and has children. Sons in fact. And his eighth son, Coin, is a sourcerer (the eighth son of an eighth son of an eighth son = a wizard squared = very powerful magic). But surely it's not that bad - it's not the end of the world, is it? Yes, it could be. The shade of Ipslore, through his sourcerer son, instigates wizard war. Hellish, apocalyptic events are set in motion. The four horsemen are abroad. The denizens of the dungeon dimension are struggling to rise. Ice giants are tearing across the plains. The gods are imprisoned. All that stands between the discworld and armageddon, is a spineless wizard, a barbarian (hairdresser wannabe), a grocer (barbarian wannabe) and a librarian ape. It doesn't look good. You shouldn't laugh ... but you will. And guess what. Rincewind, who is very well known for his complete lack of courage and over-developed instinct for self-preservation, does the most suicidally brave thing imaginable in an effort to save his beloved university library and the world.
It's another Pratchett gem. Doctors might consider prescribing these books instead of anti-depressants for some patients. They always work for me.