The Doctor and Ace land at a British Naval installation near Maiden’s Point on the Northumberland coast during World War II. From the personnel, they learn that the base, under Commander Millington (Alfred Lynch) is a listening post to intercept and translate German coded messages using the Ultima Supercomputer. There is also a stockpile of nerve gas at the base. Wheelchair-bound Doctor Judson (Dinsdale Landon) is also using the computer to decipher Viking runes in the catacombs beneath the base. The runes warn of a being called Fenric, and Millington believes he can gain Fenric’s power. Outside the base, the Doctor and Ace discover a secret squad of Soviet commandoes, led by a Captain Sorin (Tomek Bork), who wish to capture Ultima. Ace finds herself drawn to Captain Sorin.
The Doctor and Ace find a glowing Oriental vase, Viking spoils stored in the catacombs, but it is taken by Millington so he and Judson can decipher the runes. Vampire-like Haemovores emerge from the sea, attack both the British and the Russian soldiers and turn them into Haemovores. Working with Ultra, Judson is struck with energy and becomes infused with Fenric, who gives orders to the Ancient One (Raymond Trickett), a Haemovore in control of the others, who want to poison the Earth. Ace warns one of the base personnel, Kathleen (Cory Pulman), who escapes with her young child Audrey (Aaron Hanley).
The Doctor reveals that he has faced Fenric in the past—an ancient evil from the Dawn of Time—in a chess game. He sets up another game to tempt and delay Fenric, while he leaves to find a better solution. Sorin, the last Russian standing, enters the room and takes aim at Fenric, but Fenric reveals that Sorin is descended from Fenric’s Vikings and takes over his body. When Ace comes in, she helps Fenric, in Sorin’s shape, to solve the chess problem.
The Doctor arrives too late to stop Fenric, who orders the Ancient One to attack the Doctor, but Ace’s trust in the Doctor is a psychic barrier the Ancient One cannot cross. The Doctor reveals to Ace that she too is one of Fenric’s pawns, since Fenric created the time-storm that brought her to the Doctor. Audrey, Kathleen’s daughter, will grow up to be Ace’s hated mother. Ace’s faith is shattered, causing the psychic barrier to fall, but instead of attacking the Doctor, the Ancient One pulls Fenric into a sealed chamber and releases the poison gas, killing them both. As the Doctor and Ace recover on the shore, she contemplates the meaning of all this in her life, and they return to the TARDIS.
The serial was praised as magical, but thought too convoluted, with a mixed bag of special effects. It was shot entirely on location. The Doctor wards off the Haemovores by chanting the names of his companions—Susan, Barbara, Vicki, Steven, etc. Doctor Judson was based on Alan Turing, and the Ultima Machine is the Enigma Machine. Turing’s frustration with his homosexuality is transmogrified into Judson’s paraplegia, since the BBC was still not handling the subject. Two of the Haemovores are played by Sylvester McCoy’s sons Sam and Joe. The Doctor is ambidextrous and can write two texts at the same time. The names of all the Russian characters come from the works of Anton Chekhov.
Fenric is also called Aboo-Fenran, the Wolf, the Hunger, and Hastur the Unspeakable. The Doctor met Fenric in 3rd Century Constantinople and banished him to a shadow dimension in a flask. In the 9th Century, the Vikings carried the flask to England. The Haemovores are the species that Mankind will evolve into when the Earth is rotting in chemical slime after half a million years of Industrial Progress. The origin of Ingiger, the ancient Haemovore, on a dying future Earth was inspired by the 1976 film The Man Who Fell to Earth with David Bowie. The three stories referred to as the Ace Trilogy are Ghost Light, The Curse of Fenric, and Survival.