It's about Mau, an islander boy from the Pelagic Ocean. And it's about Daphne, a city-bred girl who gets shipwrecked on Mau's island. It's about a wave that destroys the Nation, about the refugees that follow, and about all of the silly and interesting things that can happen when cultures collide.
This one is probably more serious than the other Pratchett titles I've read. Still, its steeped in humor. Daphne was my favorite character, but all of them seem like they could be real. The plot clips along nicely, too. Go ahead and give it a shot.