The city of Lyonesse is sinking below the water. The water levels are to be controlled by wells. But the wells are inhabited by monsters. Idris Limpet, a peasant boy, is to be trained as a Monster Groom, a controller of the monsters. Law and order are administered by the Captains, who are both officious and somewhat unintelligent. Anyone of the human population who can swim is suspected of being a Cross, with monster blood.
Idris Limpet dives into the sea to rescue a boy, and therefore falls under suspicion. From this low point in his fortunes, aided by a girl called Morgan, he rises to become the King of Lyonesse. The name of Morgan is not the only link to Arthurian legend. The whole book may be seen as a retelling of the story of Pendragon. A light touch is added when Idris and Morgan befriend a small and not very menacing Monster named Digby.
The full significance of the Arthurian link is somewhat painfully drawn out, being clarified only at the end of the fourth and last section of the story. The characterization of Idris and Morgan and the development of their relationship form one of the book’s high points. The inner sentiments of the characters are revealed with unusual clarity, since all the humans and animals in the book can read minds. Although it has original aspects, the narrative is long and somewhat convoluted, lacking in pace.