The Doctor and Ace arrive at a naval base during World War II where an ancient evil named Fenric has been awakened. Fenric unleashes monstrous servants called Haemovores that attack the base and drain the blood of victims. The Doctor learns Fenric's ultimate plan is to poison the oceans and destroy all life on Earth. He engages in a chess game with Fenric to determine the planet's fate in an climactic finale involving the Haemovores and Fenric's possession of different characters.