While excavating the grounds of a guesthouse/farm in rural Derbyshire, archaeology student Angus Flint uncovers a prehistoric skull of what appears to be some sort of dinosaur. With the Trent sisters, Mary and Eve (who run the guesthouse) he attends a party at the home of Lord James D'Ampton commemorating the local legend of how brave knight John D'Ampton slew a giant worm and protected the villagers. While returning from the party by a country path Angus learns that Mary and Eve's parents mysteriously vanished there almost a year ago, in the vicinity of Temple House (residence of Lady Sylvia Marsh).

Upon reaching the farmhouse they are met by Ernie, the local policeman, who informs Mary that her father's watch has been found at Stonerich Cavern, a treacherous labyrinth of caves. Following Mary's information that she saw a strange car near the Temple House grounds, Ernie goes to investigate and is promptly bitten by a snake, though Lady Sylvia is able to offer assistance. During the course of his 'treatment' he informs her of the discovery of Joe Trent's watch.

The next day, while Mary and Angus investigate the Cavern and before Eve and James have returned from the party, Lady Sylvia visits the farmhouse, steals the skull and spits hallucinogenic venom over a crucifix. It is now clear that something Pagan is afoot......