In the Forest of Dean, Pierre Colet joins an esoteric community preparing its members for the collapse of urban civilisation. The experiment is financed by Simeon Marrin, who allows it to be thought that he has the secrets of alchemy. He is also an accomplished diver, who explores the bed of the Severn at night.

One night Colet joins him - and barely escapes with his life. With the growing suspicion that Marrin is secretly mining a hoard of buried treasure to finance his commune, Colet, whom his adversary believes dead, hides out in the forest and shadows Marrin's every move.