William Edward Lawrence, the central protagonist, is 10, a capable gymnast, and the new owner of a very old, very large toy castle. He receives it from his lifelong nanny, Mrs. Phillips, before her departure. William hates that she’s leaving to the point of trying to stop her, but the castle takes his mind off the loss. William’s best friend, Jason, wants to play with the castle, but William can’t bring himself to trust the other boy. Meanwhile, his parents take halting steps to become more involved in William’s life, but he’s too obsessed with keeping Mrs. Phillips around to benefit much from their efforts.

William conspires with castle resident Sir Simon to miniaturize Mrs. Phillips and confine her to the castle. Once she’s trapped, Mrs. Phillips promptly refuses to speak to the boy, who quickly realizes his scheme is ignorant and selfish. William resolves to do whatever he must to bring her back to her regular life.

He joins her as a tiny person in the castle, and later, with Sir Simon, on a grand quest to rescue a medieval county from the clutches of the evil wizard Alastor. In the process, William must look deeply into himself.