The Duc de Richleau (Christopher Lee) and Rex Van Ryn (Leon Greene) are concerned when their young friend, Simon Aaron, is absent from their annual reunion. Upon visiting him, their suspicions are confirmed when they learn of Simon's new affiliation with a so-called "astrological society" headed by the malevolent Mocata (Charles Gray). Soon, the two men discover that Mocata's society is really a devil cult taking control of innocent people through hypnotic spells and demonic ceremonies. The Duc realizes that Mocata will stop at nothing to maintain his satanic cult. The only hope lies in the success of a potent ritual that will challenge Mocata and destroy his coven forever.