Our main character is Teppic, son of the king of Djelibeybi, the Discworld's equivalent of Ancient Egypt. Teppic is sick of pyramids before he even grows to adulthood, so when he's given the opportunity, he goes abroad. To study how to be an assassin in Ankh-Morpork. But immediately after graduation, he's called home after the death of his father - a death that makes him king.
Assassin school is mildly amusing, but the majority of the book is political wrangling and pyramid building. Which Pratchett applies his usual weird humour to, but with immensely less success that usual. Teppic is a decent character, but not a great one. The last fifty pages (in which the kingdom slips sideways out of reality and is eventually returned) is somewhat more like the inspired lunacy I expect from Pratchett, but this is the least successful book in the series so far - and by a fairly wide margin.