A family settles in an old castle with plans to, among other things, turn its former cemetery into a swimming pool. However, soon they find out that, in ancient times, pagan rituals took place there involving the sacrifice of young virgins. When an evil entity starts luring young boy Mark into killing himself in order to become the next sacrificial offering, something must be done to save him. Written by Bob Baker and Dave Martin who wrote many episodes of Dr. Who.