The Doctor, Romana, K9, and now Adric (Matthew Waterhouse) land on a planet in E-space with a feudal society. The villagers live under the brutal rule of three lords—Zargo (William Lindsay), Camilla (Rachel Davies), and Aukon (Emrys James), who live in a dark tower. Every year “The Selection” takes place, in which young villagers are taken to the tower by chief guard Habris (Iain Rattray) and are never seen again. The time-travellers find old evidence of high technology and wonder about the State of Decay of the planet. The Doctor and Romana are seized by cloaked figures. Adric is seized by the owners of the Feeding House in the village, and Lord Aukon, on inspection, believes he is perfect for the Selection.
The cloaked figures are a resistance movement, who take the Doctor and Romana to a secret base filled with high tech. Kalmar (Arthur Hewlett) is a banned scientist. The Doctor helps him repair a computer which reveals the names and faces of the original officers of the spaceship Hydrax—the Lords of the Tower. The Lords have learned about the Doctor and Romana, and Lord Aukon sends an army of bats after them. They are seized by Habris and his guards and taken to the tower. Zargo and Camilla entertain them, but then must leave to deal with the Arising. The tower, of course, is the spaceship, which is from Earth and trapped in E-space like the TARDIS.
The Doctor and Romana find drained corpses and stores of blood. In an amphitheatre with a floor pulsing like a heartbeat, Lord Aukon invites them to be the Servants of the Chosen Ones, but they refuse. Zargo and Camilla take them as future meals for the Great One. The Doctor figures out that Zargo was once Sharkey, Camilla was once MacMillan, and Aukon was O’Connor—all members of the original crew, mutated into vampires that lord it over the descendants of the rest of the crew. He deduces that the Great One is known to the Time Lords as one of their ancient enemies the Great Vampires, escaped into E-space. A rebel named Tarak (Thane Bettany) penetrates the tower and frees the Doctor and Romana. The Doctor returns to the TARDIS and Romana, aided by Tarak, searches for Adric. Trying to snap Adric out of his trance, they awaken Zargo and Camilla. Zargo kills Tarak, Adric knifes Zargo, who is not hurt. The vampires advance on Romana and Adric, but Aukon intervenes. He has the power of the Great One. He wants Adric as the Chosen One and Romana—of course—for sacrifice at the Arising. In the throne room/control deck, Adric announces that he wants to be a Chosen One (a vampire) and they are taken to the bottom of the tower.
In the TARDIS, the Doctor and K9 discover the Great One can be killed by Rassilon-designed scout ships, like steel bolts, through the heart. He scans the tower and finds a demonic presence, with bat-like wings—the last Great Vampire—deep beneath it. The villagers agree to help defeat it. With K9’s help, they assault the tower. Adric reveals he was faking compliance but is unable to attack Aukon and free Romana. The Doctor launches a scout ship into the heart of the Great One. Romana comes out of her trance. The vampires crumble into dust, as they do. The Time-travellers leave the villagers to develop and advance on their own.
Fans have praised the SF/Gothic feel of the story, obviously based on vampire lore, and the strong, ruthless villains. The script, not surprisingly, had been kicking around since an earlier Tom Baker era, under Hinchcliffe, which produced a number of good Gothic stories. Even K9 is used well, showing off his powers. Tom Baker and Lalla Ward were in the throes of divorce.