Night Mare centers around Mare Imbrium, one of the night mares charged with delivering bad dreams to the people of Xanth. Imbri carries half a soul, her fee for being the unwilling steed of the half-nymph Tandy. She is unwilling to relinquish her soul, though the conscience that comes with it impedes her ability to deliver bad dreams. The Night Stallion, ruler of the gourd realm, banishes Imbri to the day world and sends her to meet Trent, King of Xanth, with an ominous message. Mare meets the relentless, unforgiving Horseman and for the night mare, everything begins to become a horrible nightmare!