As William turns twelve, he wonders if Jason is still his best friend. In the past year, Jason has grown a foot taller, while William is still a shrimp. When Jason challenges William to "jump the trains," William is terrified. How else can he prove himself to Jason? William gets his answer when his former housekeeper sends him a magic token that once again transports him to Sir Simon's castle in the Middle Ages, this time with Jason. There the two friends face a dangerous army of rats and their gigantic leader. Once before, William was a hero in Sir Simon's day. Now it's up to William to destroy the rats and save the kingdom in the final battle for the castle.