Cynthia Centaur and her companions set out to collect the Six Rings of Xanth — one each for Air, Earth, Fire, Water, Void, and Idea — as the only way to free a captive demon from a rival's grip. Assembling them means crossing most of Xanth and calling in favors from old friends along the way. The twenty-fifth Xanth novel builds its quest around the series' cast of feuding, cosmically powerful demons.
Summary by Loreful, from public sources