Set in London and the south of England in 1929, the story finds Duke de Richleau (Christopher Lee), investigating the strange actions of the son of a friend, Simon Aron (Patrick Mower), who has a house replete with strange markings and a pentagram. He quickly deduces that Simon is involved with the occult. Nicholas de Richleau and Rex Van Ryn (Leon Greene) manage to rescue Simon and another young initiate, Tanith (Niké Arrighi), from a Theistic Satanismping cult. During the rescue, they disrupt a ceremony on Salisbury Plain, in which the Devil, the "Goat of Mendes" (Baphomet) appears.