The Lambton Worm has waited for centuries for some one to come along with evil deeds or thoughts to set it free. One Sunday a lord's son sneaks off to fish, thinking angry thoughts about his little sister.
Seriously? Thousands of years and the most evil thing that has happened in the area is a kid fishing on Sunday instead of going to church, and being mad at his sister for threatening to tell on him? That's kind of implausible.
But anyway, the boy traps the evil worm in a well. Then he goes home and is immediately knighted and sent off to the Crusades. His sister sneaks along as his squire. But along the way they get chased by robbers and then held for ransom by a wicked nobleman. Eventually the boy is rescued by his sister and returns home to battle the worm, which has been doing super-evil stuff like stealing milk and pies.
I realize this is for little kids and the author probably didn't want to make it too scary, but why keep using the word EVIL when really the worst that happens is naughtiness? Also, the whole trip-to-the-Crusades part seems kind of unrelated to the Worm. And it wasn't too believable that the wealthy parents send the boy alone and with no advice to travel abroad.
The best aspects of this book were the Worms humorous dialog and the spunky sister.