England, the 1950s. Father Michael Rainer is excommunicated from the Catholic Church for refusing to recant his heretical beliefs.

1970s London. John Verney, a renowned occult writer, is approached at an art gallery by Henry Beddows, who asks for his expert help. Beddows is trying to extricate himself and his daughter Catherine from the influence of a Satanic cult. Verney agrees to assist him, sensing the possibility of material for a new book.

Catherine Beddows, a young nun, arrives at Heathrow Airport from Germany. Verney sneaks her away from her bodyguard and takes her to his apartment. He learns that she is a member of a sect called The Children Of Our Lord. She has been kept isolated all her life in a Bavarian convent, only seeing her father once a year on her birthday. Her only friends are Father Michael, a pregnant woman called Margaret, and her foster parents, George and Eveline. Verney notes that her birthday is the next day, All Hallow's Eve.

The bodyguard heads for Beddows' manor on the orders of Father Michael. There is a struggle, and Beddows shoots the man dead.

Back in Germany, Catherine's friend Margaret goes into labour. Father Michael ties her to a bed and binds her legs together. The delivery seems more like a religious ceremony than a medical procedure. The baby tears its way out of Margaret's stomach. Father Michael places the monstrous creature in an incubator, and then kills Margaret with a morphine overdose.

Father Michael, George, Eveline and the baby secretly fly to England. Upon arrival, Father Michael rings Beddows and uses magic to make him reveal Catherine's whereabouts. He reminds Beddows of the pact he made eighteen years ago when Catherine's mother died in childbirth. If he disobeys, Beddows will die by fire.

Verney goes in search of information about The Children Of Our Lord, leaving his friends David and Anna to watch over Catherine. Father Michael uses magic to contact Catherine, who at his command opens the birthday present given to her before leaving the convent. The gift is an inverted crucifix of a spread-eagled young man, an image of the demon Astaroph. Once she dons the crucifix, Catherine comes fully under Father Michael's power. Verney catches her just before she can escape.

When Verney removes the crucifix, Catherine has a vision where she sees Margaret having ritual sex with Father Michael. At different points, Catherine imagines that it is she, not Margaret, who is being violated, and that the sex acts are being carried out by a giant golden effigy of Astaroph.

Father Michael rings the flat, demanding that the girl be handed over to him. Verney leaves Catherine with David and Anna, and sets off to gather the necessary information to defeat Father Michael.

While Catherine sleeps, Father Michael takes control of her once more. She kills Anna with a steel comb and escapes from the flat. Meanwhile, Verney has gained access to the Grimoire of Astaroph, kept in a secret library by the Catholic Church. He learns that Father Michael plans to baptise Catherine with the blood of Astaroph, turning her into the living avatar of the demon.

With Catherine now missing, Verney realises that Beddows is the only person who can help him stop the baptism. He and David find Beddows cowering inside a protective circle. He promises to tell Verney what he knows if he can break the pact he has made, which is inscribed on an amulet hidden in the local church.

Verney retrieves the amulet, but the guardian spirit of Beddows' dead wife burns David to death. Freed from the pact, Beddows directs Verney to a nearby mausoleum, where he finds that Eveline has sacrificed herself to create a protective circle of blood around Father Michael and Catherine. Verney finds George standing guard and kills him with a sharpened stone. He hurls the bloody stone at Father Michael just as he cuts the throat of Margaret's demonic baby. The stone counteracts the protective circle and Father Michael is destroyed by the forces he has unleashed.

Verney carries the catatonic Catherine from the mausoleum. He doesn't notice the drops of blood on her forehead.