When a faun's nymph friend vanishes into the Void, her bonded tree starts to wither — and saving it means finding a replacement faun before it's too late, at the risk of losing his own tree in the process. The Good Magician sends young Forrest Faun and the shapeshifting mare Imbri off to the small world of Ptero and beyond, chasing a fix across moons that shouldn't exist. The twenty-first Xanth novel widens the map past Xanth itself.
Summary by Loreful, from public sources