In a startling twist of fate, Father Michael Rayner, a once steadfast Catholic priest, faces excommunication due to his alleged heretical views. Fast forward two decades, and he emerges as the elusive leader of the Children of the Lord, a clandestine religious order located on a secluded Bavarian island. Among the initiates is Catherine Beddows, a girl who has spent her entire youth within the convent walls, thanks to her late mother's fervent devotion to the order's cause.
As Catherine's eighteenth birthday draws near, Father Rayner decides to release her from the stringent confines of the cult, signaling the beginning of a new chapter for her. Meanwhile, John Verney, a celebrated American writer and expert in occult matters, is approached by Henry Beddows, Catherine's father, during a book launch event. Desperate and worried, Henry seeks Verney's assistance to bring Catherine back from Heathrow Airport, convincing him that the Children of the Lord is, in fact, a nefarious Satanic cult intent on corrupting innocent lives.
Intrigued yet cautious, Verney agrees to this peculiar request. Upon arriving at the airport, he spots Catherine in a habit and successfully intervenes just in time to save her from a cunning cult member who tries to reclaim her. Bringing Catherine to his home, Verney encourages her to share her harrowing experiences within the cult.
Just as they begin to bond, Henry's urgent phone call interrupts their conversation, seeking permission to pick up Catherine. However, the call is abruptly cut short by the same cult member, now brandishing a gun, attempting to impede Henry's rescue efforts. In a separate scene, Father Rayner and a nurse help Margaret, a pregnant cult member, as she approaches the moment of childbirth. After the delivery, Rayner administers a lethal dose of morphine, ending her life cruelly.
The next day, Verney confides in Anna and David about his ominous suspicions that the cult plans to use Catherine in a malignant ritual aligned with her upcoming birthday, coinciding with All Hallows' Eve. He leaves Catherine in their safe keeping, setting out on a quest to uncover the London headquarters of the Children of the Lord, only to find it has been transferred to a Christian organization.
The cult, desperate to reclaim Catherine, resorts to dark sorcery to pull her back into their fold. She feels inexplicably drawn to the St Katharine Docks, where Verney fortuitously comes across her, interrupting the cult's ominous plans by tearing the sacrilegious pendant, depicting Astaroth, from her neck. His warning words trigger a deluge of memories in Catherine, unveiling her traumatic experiences and the involvement of Father Rayner in the disturbing sex magic rituals that led to Margaret's pregnancy.
As Verney digs deeper into the mystery, he uncovers a malevolent scheme contrived by Rayner, who aims to exploit Catherine's unwitting allegiance to channel the dark powers of Astaroth. The bishop's cryptic remarks reveal a chilling truth: the cult's objective is to transform Catherine into a vessel for the demon, with Rayner believing this unholy union will grant him unbridled power and a conduit to the true God. Under Rayner's manipulative grasp, Catherine succumbs to a horrific act of violence against Anna, wandering the city streets in a trance-like state, irresistibly drawn back to the cult that has birthed Margaret's demonic offspring.
As the tension heightens, Verney and David seek to find Henry, whose agreement with Rayner is precariously at stake. Discovery of Catherine's mother's ghostly apparition rising from the church altar, where the accursed pendant is concealed, leads to disastrous consequences. In a desperate attempt to retrieve it, David meets a fiery end, while Henry narrowly evades a similar fate.
Undaunted, Verney tracks Rayner to an abandoned mausoleum yard, where he finds Catherine laid out on an altar, encircled by a grotesque ring of human blood donated by Eveline. After witnessing Rayner's unspeakable sacrificial act involving the demonic infant, Verney stands up to his mentor, condemning his warped understanding of Astaroth's grimoire and cautioning him against the perilous path he has taken. Convinced of his own divine mission, Rayner offers Catherine to Verney, who retreats from the tempting offer.
Using a blood-infused rock acquired from a fallen security guard, Verney breaches the protective circle that surrounds Rayner's sacrilegious ritual. With determination and precision, he hurls the rock at Rayner, casting him into oblivion. The chaos subsides as Verney rescues Catherine, carrying her away from the now-disbanded blood circle. Although she has been partially immersed in the ritual's blood, leaving her fate uncertain, Verney's timely intervention grants them a narrow escape from the grip of Rayner's dark machinations.
