When Cynthia Centaur journeys to ask the Good Magician Humfrey a very personal Question, she expects nothing more than the usual Challenges and Puzzles, as well as some small sort of Service, in exchange for her Answer. What she doesn't expect is that she will be responsible for the future survival of Xanth itself.
For a peril of immense gravity endangers the enchanted land of Xanth. Or, rather, a lack of gravity. The Demon Earth, who rules our own world, has suddenly and mysteriously disappeared. Without Demon Earth's vast powers, everything on the surface of the Earth and all other worlds, including Xanth, will go whirling weightlessly away into space.

By merest chance, Cynthia Centaur has come to the Good Magician's castle at precisely the wrong moment. For the Muse of History has decreed that the very next Querent who asks Humfrey a Question will be charged with the awesome responsibility of finding the Demon Earth and saving Xanth.

It's whispered that another Demon (with a capital D) has done the dastardly deed. The only weapon that can prevail against such a cosmic being is the instrument known as the Swell Foop. And the only way to handle its awesome power is to locate the Six Lost Rings of Xanth, whose whereabouts are known only to the Zombies.

So Cynthia and five companions-her beloved mate, Che; Breanna of the Black Wave and her fiancé Justin Tree; Sim, the immortal bird who will one day know all the secrets of the universe; and Jaylin, a young human girl from the Mundane land of Hawaii -embark on a quest of utmost danger, to challenge an Unknown Demon for the fate of a dozen worlds.
A harrowing and hilarious escapade that ranges from Earth to Xanth to a galaxy far, far away, Swell Foop is pure magical merriment from the prolific pen of fantasy master Piers Anthony.