American expatriate occult writer John Verney (Widmark) is asked by Henry Beddows (Elliot) to pick up his daughter Catherine (Kinski) from London airport. Catherine is a nun with the Children of the Lord, a mysterious heretical order based in Bavaria and founded by the excommunicated Roman Catholic priest Michael Rayner (Lee), where Beddows is allowed to come to visit Catherine only on her birthdays. But after Catherine arrives, Beddows then insists that she stay with Verney. The order, however, under Father Michael, makes all efforts to get Catherine back and uses black magic to stop Verney as he protects her. Verney learns that the order really harbours a group of practicing Satanists, who have prepared Catherine to become an avatar of Astaroth upon her eighteenth birthday. The priest kills Verney's friends, and tries to get Verney. Verney battles the priest and his henchmen and is able to rescue Catherine.