The big wave took everything Mau has ever known - his Nation (a tropical island) has been swept away, along with all its inhabitants. The young man lived only because he was on the water when it happened.
The same wave killed everyone aboard a large sailing ship - except one properly raised "princess" ill-equipped for primitive survival. Against the odds and a language barrier, together they survive, grieve, grow, learn to communicate, and build a new Nation with the refugees who appear one by one on the island.
As with any Pratchett book, there's a lesson, some philosophical questions to ponder, and a lot of just really funny gags. This one pokes fun of monarchy and manners, introduces us to the tree-climbing octopus, and introduces a new brand of Robinson Crusoe.