Prospero and his friend Roger Bacon, both wizards, embark on a journey to save humankind from the evil spell of their former colleague, Melichus. Melichus has found an old book, written in curious ciphers, which provides glimpses of some basic formulas of power when he tries to translate the strange writing. The more Melichus pursues the translation, the more entranced he becomes; what begins as intellectual curiosity quickly becomes an obsession with the power the spells might bring him. The formulas contain the secret to another reality, and whoever masters these arcane symbols holds the power to destroy the North and South Kingdoms. Prospero and Bacon strive to prevent this at any cost.
The mark of Melichus growing power over the land is the recurring image of the face in the frost, a yawning, vacant visage that evokes a nameless terror that cannot be dispelled by reason. Prospero and Bacon begin their journey by shrinking themselves and sailing a model ship down an underground stream leading from Prosperos root cellar. During their adventures, the magicians defeat a troll, use a prophetic looking glass, ride in a squash coach, repel spells cast by an enchanted forest, and climb a magical vine to reach the fairy-tale cottage Prospero and Melichus once shared in happier days. These familiar images from myths and fairy tales weave a rich tapestry of allusion throughout John Bellairs tale.
At the cottage, Prospero retrieves a green glass paperweight that contains the combined powers of Prospero and Melichus. Prospero enters the world of the prism, a strange place in which technology has prevailed. It is filled with electricity, lawnmowers, and the accouterments of modern civilization. He encounters M. Millhorn, a believer in the occult who has been waiting all his life for this moment. In exchange for the paperweight, Millhorn uses his knowledge of the Kabala to save Prospero and return him to his own world. Prospero finally remembers the spell he must use to destroy Melichus and the evil book. Prosperos world returns to normal, and he and Bacon celebrate their triumph with a party for their friends.