Based on the 1953 novel of the same name by Dennis Wheatley, it follows an American occult researcher in England who attempts to save a young girl preyed upon by a Satanic cult led by a fallen Catholic priest.

Twenty years later, Rayner oversees the Children of the Lord, a mysterious religious order headquartered on a small island in Bavaria.

Meanwhile, John Verney, a famous American writer and occult expert residing in London, is approached at a book release party by Catherine's father, Henry, who asks Verney to pick up Catherine from Heathrow Airport and temporarily entrust her in his care, claiming the Children of the Lord is in fact a Satanic cult from which he wishes his daughter to be saved.

The next morning, Verney is visited by his friends Anna and David, and informs them he believes that the cult intend to use Catherine in a ritual on her birthday the following day, which happens to be Halloween.

Verney leaves Catherine in Anna and David's care while he attempts to locate the London headquarters of the Children of the Lord, only to find it has been sold to a Christian organization.

She visits the St Katharine Docks, where Verney happens upon her, sabotaging the cult's efforts by ripping a sacrilegious Astaroth pendant from her neck.

Verney attempts to convince the impervious Catherine that Astaroth is evil, triggering memories of the sex magic rituals the cult subjected her to, during which Rayner impregnated Margaret.

Meanwhile, under Rayner's influence, Catherine murders Anna before wandering the city streets, only to be led directly to the cult, who have welcomed the birth of Margaret's demonic infant child.

[9] The television project never materialized, but actor Christopher Lee championed the novel, convinced it would make an effective feature film.

[10] Ken Russell and Nicolas Roeg were considered to direct the project, but Australian director Peter Sykes was ultimately hired.

[10] Shooting locations included Heathrow Airport, the St Katharine Docks,[10] and the Dashwood Mausoleum and Hellfire Caves in West Wycombe in Buckinghamshire.

[19] Actor Richard Widmark was allegedly displeased with the production during shooting, and several times threatened to leave and return to the United States.

[9] Variety called the film a "lacklustre occult melodrama" that "seems padded and tentative, and though horrific in spots the actual shock value is remarkably subdued.

"[30] Linda Gross of the Los Angeles Times found the story "a confusing vacillation between special effects, hallucinations, psychic trances and ongoing narration," but thought the film was "distinguished by engrossing performances," "superior photography" and "eerie music.

"[25] Gary Arnold of The Washington Post felt the film was poorly-made, writing that it "seems to have been scripted, directed and edited with extreme haste and negligence, as if the filmmakers had to keep one step ahead of process servers or the finance company.

"[31] Critic Roger Ebert echoed a similar sentiment, writing that, "The screenplay has so many characters, and they're in so many different places, that the only way to keep them halfway straight is for them to be calling each other all the time.

"[32] Tony Rayns of The Monthly Film Bulletin praised the "expert special effects" and "no-nonsense script," and commented that Christopher Lee played his role "with a gusto absent from his performances for many years.

It feels as dangerous as it should, built on performances that resonate and visuals that stick in the mind's eye long after the credits complete their elegy.