Set in 1930s London and the South of England, Duc de Richleau and Rex van Ryn rescue their friend Simon Aron from a devil-worshipping cult. Rex falls in love with another initiate of the cult, Tanith. Rex prevents Tanith from going to a ceremony on Salisbury Plain. The Duc and Rex rescue Simon from the ceremony. They escape to the home of the Eatons, friends of Richleau and van Ryn, and are followed by the group’s leader, Mocata, who has a psychic connection to the two initiates. After visiting the house to discuss the matter and an unsuccessful attempt to influence the initiates to return, Mocata forces Richleau and the other occupants to defend themselves through a night of black magic attacks. During this Mocata summons the Angel of Death using the medium of Tanith. The defeat of the Angel results in Tanith’s death.

After successfully defending themselves through the night the group find that Mocata has kidnapped the Eatons’ daughter. Simon exchanges himself for her. Mocata is using Simon to find the Talisman of Set, a powerful satanic object. The book culminates in a desperate chase across Europe to an abandoned Greek Monastery where Mocata is defeated. The group wake up in the Eatons’ home and realise that during the ceremony they entered the fourth dimension. Mocata is found dead outside the house. The Duc wakes up clutching the Talisman and destroys it. Tanith is found to be alive - Mocata’s soul has been exchanged for hers.

In 1967 the novel was made into a film by the British film company Hammer Film Productions. It starred Christopher Lee as de Richleau and Charles Gray as Mocata.