Two years have passed since Peter Nimble and Sir Tode saved the kingdom of Chestnut Isle. In the meantime, the two friends traveled to the ends of the world in search of adventures. But now Professor Cake calls them to assign them a new mission: to find a twelve-year-old book restorer named Sophie Quire.
Sophie spends most of her time within the four walls of her father's bookstore, repairing old books, while dreaming of escaping the narrow confines of her monotonous life. But then she is visited by a strange boy and his companion, a creature that is a combination of cat and horse who can speak, presenting her with a rare and mysterious volume, and drawing her into an adventure that surpasses any story she has ever read in books.