The Gellhorn twins, Maria [Mary Collinson] and Frieda [Madelaine Collinson] have journeyed from Vienna to Karnstein in order to live with their aunt and uncle, Gustav [Peter Cushing] and Cathy [Kathleen Byron] Weil, fo...展开llowing the death of their parents. Gustav Weil is a severely puritanical witch hunter who spends his nights searching out and burning the devil's servants--suspected witches and vampires. One of his greatest arch-enemies is Count Karnstein [Damien Thomas], said to practice the black arts and worship the devil, all under protection of the emperor. Karnstein is becoming weary of tedious black masses and bogus manifestations of the devil; he wants the real thing. When he becomes bored with the current 'sacrifice' of a village virgin, he completes the sacrifice himself. The blood seeps through a crack in the table, which just happens to be the tomb of Marcilla Karnstein [Katya Wyeth]. Marcilla resurrects and makes Karnstein vampire, too. Aunt Cathy enrolls the girls in school where the choirmaster Anton Hogger [David Warbeck] takes an immediate liking to Frieda. However, Frieda finds Count Karnstein more fascinating. One evening when Frieda has had enough of Uncle Gustav, she sneaks out of the house. She meets with Count Karnstein's carriage and is taken to the castle where he makes her vampire.
Meanwhile, Anton Hoffer writes a letter to the church elders complaining about Gustav's burnings. "By burning," Anton scolds, "you char the body. The soul will only re-create itself in another body and continue with its carnage. Only a stake through the heart or decapitation will end their tormented evil." For all his knowledge, however, Anton's sister Ingrid [Isobel Black] is found the next day with vampire bites on her neck.
Then it happens that Gustav comes across his own niece Frieda vampiring a man. "The Devil has sent me twins of evil!" he cries. He sends Frieda off to jail and goes home for Maria. Aunt Cathy will not allow him to take Maria who, at this very moment, is lying upstairs with a cross clasped to her breast. Unfortunately, Maria has a nightmare and drops the cross. Karnstein abducts Maria and switches her with Frieda. While the witch hunters prepare to burn Frieda (actually Maria), Anton goes to see Maria and recognizes Frieda right away. He escapes to warn Gustav who does not believe him.
"Hold up the holy cross and see if either of us flinches," challenges Anton.
When both Maria and Anton pass the test, the puritans storm Karnstein castle to destroy Karnstein and Frieda. Gustav beheads Frieda himself, but Karnstein siezes Maria and threatens to throw her from a balcony. Gustav attempts to behead Karnstein with an ax but he misses; Karnstein axes Gustav instead. As Karnstein watches Gustav die, Anton throws a spear which stakes Karnstein through the heart. Maria is saved. [Original Synopsis by bj_kuehl.]