In 1920s England, the suave and knowledgeable Duc de Richleau discovers that his young friend, Simon Aron, has been initiated into the dark and mysterious world of a Satanic cult led by the charismatic Mocata. Worried for Simon's safety and soul, Richleau learns that the cult intends to summon the powers of darkness to achieve their nefarious ends. Accompanied by his skeptical but loyal friend Rex Van Ryn, de Richleau attends one of the cult's gatherings at Simon's country house under the guise of interested recruits, only to confirm his worst fears. A daring rescue ensues as they whisk Simon and another young initiate, Tanith, away from the clutches of the cult, seeking refuge in a protected circle that Richleau creates with his deep knowledge of the occult.

The cult, however, does not easily relinquish its prey. Mocata exerts his diabolical influence, launching a series of supernatural assaults on the small group to reclaim Simon and Tanith. As they struggle to evade Mocata's malevolent forces, Richleau employs ancient rituals and symbols to counter the dark magic, fully aware that a single misstep could spell disaster for all involved. The confrontation becomes a harrowing battle of wills between the forces of good and evil, each side armed with arcane knowledge and unwavering conviction. The fight intensifies as the group races to thwart a dire prophecy before the imminent rise of a powerful demonic force.