TO THE DEVIL A DAUGHTER

Father Michael Rayner, a Catholic priest, is formally excommunicated from the church for heresy.

Twenty years later, Rayner oversees the Children of the Lord, a mysterious religious order headquartered on a small island in Bavaria. Catherine Beddows, a teenage girl, has been raised in the order at the urging of her late mother, who was a dedicated member. Catherine is formally released from the island convent by Rayner days before her eighteenth birthday.

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. While Verney's literary associates find the request strange, Verney agrees.

Upon arriving at the airport, Verney finds Catherine, who is in a habit, and manages to intercept her from a member of the order who is escorting her. Verney brings Catherine back to his home, where he gleans information about her upbringing in the cult.

Henry phones Verney's residence and tells Catherine he will come to get her, only to be interrupted by the member who escorted her; wielding a gun, he attempts to stop Henry before Henry shoots him dead. Meanwhile, at a private residence, Rayner, along with a nurse, oversees the pregnant Margaret, a member of the order who is going into labour. After she gives birth, he has her killed with a morphine injection.

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.

Meanwhile, the cult use black magic in an attempt to draw Catherine to them. She visits 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.

Verney gleans from the bishop who excommunicated Rayner that the cult are aspiring to prepare Catherine to become an avatar of the demon Astaroth, whom Rayner believes to be a route to perfecting human power, and thus the true God. 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.

Verney and David visit a dishevelled Henry, who directs them to a church to locate the metal "pact" he made with Rayner, which threatens Henry's life. The two men find the ghost of Catherine's mother rising from the church altar where the "pact" pendant is hidden. David bursts into flames and burns to death upon touching the pendant, though Henry is saved.

Verney tracks Rayner to an abandoned mausoleum yard where Rayner has laid Catherine on an altar and encircled it with human blood from another cult devotee, Eveline, who gives her blood, and life, for the purpose. He bludgeon's George, who is guarding the area, with a rock and then witnesses Rayner sacrifice the demon child in preparation of baptizing Catherine to become Astaroth's avatar. Verney confronts Rayner, critiquing his beliefs and warning Rayner that he has misinterpreted the grimoire of Astaroth. Rayner asserts his confidence, and offers Catherine, who appears as a nude spirit, to Verney, who refuses the temptation.

Verney uses the rock which is covered with George's blood, as a talisman to defeat the protection of the circle of blood. Then Verney throws the rock at Rayner, vanquishing him; Rayner vanishes into nothingness. Verney manages to save Catherine, carrying her away, as the two flee Rayner's circle of blood. However, Catherine has been partially baptized with several drops of blood, leaving her status in question.

John Verney ultimately manages to stop Father Michael Rayner and the cult from using Catherine as the avatar of the demon Astaroth, saving her from their Satanic plans.

The film’s conclusion sees the occult expert Verney successfully intervening in the cult’s ritual, preserving Catherine’s humanity and halting Rayner’s scheme to usher in a demonic entity through her. The ending underscores the recurring Hammer theme of rationality and human courage triumphing over supernatural evil, while also highlighting the vulnerability of innocence in the face of fanatic devotion.