Professor Ewart Masters' convalescence takes a terrifying turn when his studies of ancient civilizations lead him to uncover a hidden city beneath the Yorkshire moors. While recovering from a car accident at his nephew Jason's home, Masters becomes engrossed in deciphering Robert Krug's manuscript and the mystery of strange green figurines. His investigation draws him towards the ominous Devil's Pool and the forgotten world of Lh'yib, plunging him into a series of dream-like, nightmarish adventures within this subterranean realm.

The novel is characterized by its Gothic atmosphere, blending brooding mystery and stark terror with moments of quiet charm and subtle humor. The author employs a straightforward, almost autobiographical narrative technique, which lends an immediate quality to the unfolding events. This approach is noted for achieving a sense of immediacy seldom found in this genre. The narrative structure allows for a deeply immersive experience of the protagonist's descent into the unknown.