The Devil Rides Out features “The Four Musketeers”—Duke de Richleau, Rex Van Ryn, Richard Eaton, and Simon Aron—series characters who appear in Dennis Wheatley’s political thrillers as well as his black magic novels. Alarmed by the fact that their friend Simon Aron has fallen under the influence of a black magician, Damien Mocata, occult expert Duke de Richleau and his friend Rex Van Ryn set out to help him. They learn that Aron is the key element in the magician’s conspiracy to activate the powers of the Talisman of Set, thereby giving Mocata control over the Four Horsemen of the Apocalypse. They set out to free their friend and save humanity.
The first rescue is relatively easy, but Aron is recaptured. In a heroic Walpurgis Night assault on Mocata’s castle, the duke and Van Ryn again rescue Aron. They spirit him to Cardinals Folly, the estate of Richard Eaton, where they are all safely harbored against Mocata’s demoniac attacks in a specially prepared magic pentacle. Temporarily frustrated, Mocata launches a counterattack on the duke’s group by kidnapping Fleur d’Amour, Eaton’s innocent young daughter. Her ritual sacrifice will loose the powers of the Talisman of Set for Mocata. The Four Musketeers, accompanied by Fleur’s mother, Marie Lou, race to Paris for a final confrontation with the black magician. The men are stymied by Mocata’s occult defenses but, moments before the sacrifice is to occur, Marie Lou recalls a magical dream and utters words that tear the talisman away from Mocata and summon the Lord of Light. Taking over Fleur’s body, the Lord of Light turns Mocata’s own demons against him in a final battle that takes place on the Astral Plane.