In an alternate universe, a plague ravages the known world, and leaves a duke who was 139th in line for accession King of England. Of course, he doesn't realize this because...
The heroine - a young girl cursed with the name Ermintrude is off on adventure at her father's behest. A tsunami hits, crashes the ship onto an island, and kills everyone except for her...and a parrot. She survives because she wrapped herself in a mattress.
The hero - a young man/boy (age not hinted at) is headed back to the island his people inhabit when the wave hits. He makes it back to find everyone gone... and a young girl stranded.
Together, they forge what amounts to a refugee camp for survivors from other islands. They also learn a lot about themselves, and the past.
Eventually, the girl's father finds them. Shortly thereafter, the Crown finds them. The heroine goes home.
There is a sense of a Lord of the Flies element here - without the sociopath. Or rather, the sociopath is an adult who is defeated by a boy. More importantly, nobody actually likes the sociopath. (BTW, I never liked LOTF - the fact that so many were able to completely abandon their conscience bothered me a lot).
The epilogue is set in modern times, where the descendants of the refugees still live on the island, which has become a mecca of sorts for scientists everywhere.
I am...more familiar with Mr. Pratchett's somewhat offhand humor so this decidedly serious book was a bit startling. I liked it as far as that, but had I known of its unfunny nature, I probably wouldn't have picked it up.