Cult horror picture directed by Jesús Franco in which laid-back Kathleen (Anne Libert) and devout Margaret (Britt Nichols) are two orphaned sisters living in a convent. The evil Lady De Winter (Karin Field) torments the two girls and is convinced that they are daughters of a witch who was burned at the stake. In the belief that they will have their revenge for their supposed mother's death, De Winter sends Kathleen to a witch-hunter to be tortured and burned to death. As Margaret is so profoundly religious she is kept safe, but when she hears of her sister's plight she succumbs to the devil in a lecherous way before wreaking Satanic havoc on the house's occupants...