Continuing the Doctor's adventures in E-Space, the Doctor, Romana, K-9, and their newest companion/stowaway, Adric, arrive on a planet experiencing what appears to be a feudal period. The population scratches out a living as subsistence farmers under the thrall of three local lords, Zargo, Camilla, and Aukon, who dwell in a shadowy tower. Adding further fear to their lives, they experience a yearly ritual called "the Selection," in which a sample of young villagers are taken to the tower, never to be seen again. This selection process is enforced by a thuggish band of guards led by Habris.
The Doctor and Romana venture out into the village (not knowing that Adric is following them), and it doesn't take long for the Doctor to realize that something is very wrong when he discovers evidence of technology considerably more advanced than what this medieval society seems capable of producing. With such technology, the Doctor and Romana wonder what happened in the course of the planet's development to cause it to evolve "backward" from a presumably advanced culture to its current rustic condition -- to be in a "state of decay." The arrival and then departure of the Doctor and Romana from the village hall is reported by the headman, Ivo, via an electronic communications device to an unseen figure called Kalmar. As the two head out of the village they are seized by cloaked figures who convey them to a secret base filled with illegal computers and other technology. Kalmar is a scientist - a heretical role in their society - and is very grateful for the Doctor's help in repairing a computer which proceeds to reveal the names of the original chief officers of the spaceship Hydrax. The faces of three senior officers are revealed as those of the Lords of the Tower.
The Lords too have learnt of Romana and the Doctor, and Aukon sends a flock of his winged servants, bats, (aka "The Wasting") to menace them as they travel a clearing near the village.
The Doctor and Romana are now seized by Habris and his guards and taken to an audience in the Tower. Zargo and Camilla entertain them for a while, then are called away to deal with a situation called the Arising. The Doctor and Romana start to snoop around and discover that in fact, the great Tower in which the Lords dwell is itself a spaceship originally from Earth, which also somehow was pulled into E-Space long ago.
Adric has meanwhile wandered into the village and finds himself looked after by Ivo and his wife Marta, both of whom are grieving for their late son who was chosen for the Selection.
In the Hydrax the Doctor and Romana discover rows of corpses drained of blood, while the craft's fuel stores are full of blood. Talk turns to vampires. They head downwards and find an amphitheatre. It is there that the Lord Aukon greets them, welcoming them to his domain.
Aukon invites the Doctor and Romana to become the first of the new servants of the Chosen Ones then tells them that he has Adric much to their surprise. When they refuse they are imprisoned. The Doctor deduces by applying principles of consonant shifting that the current lords' names are a corruption of the original crew names (e.g. "Sharky" becomes "Zargo"). Thus the Doctor realizes that the three lords might not be descendants of the original crew, but members of the original crew themselves. He is reminded of ancient Time Lord stories of the Great Vampires, a giant race of rapacious, destructive, and powerful creatures that were ancient enemies of the Time Lords themselves. He deduces that the Great Vampire escaped destruction at the hands of the Time Lords by somehow retreating into E-Space, and it managed eventually to gather enough power to pull the old Earth ship into this universe and corrupt the crew. Over time, it licked its wounds and gathered power so that it could once again feast on worlds.
Meanwhile, Adric's attempts to mingle with the natives have led to him getting caught up in the Selection. He is put under a hypnotic trance by Aukon and accompanies him to the tower. Normally, the Chosen Ones are chosen for Guards or killed if they defy the Three Who Rule, but Aukon has other plans for Adric when he finds, to his surprise, something different about the third stranger. Aukon plots to change Adric into the new Chosen.
Another rebel named Tarak, makes a solo attack on the Tower in the hope of freeing the Doctor and Romana. The Doctor returns to the TARDIS, while Romana stays with Tarak to search for Adric, whom she knows now is in the Tower's Inner Sanctum. As they try to snap Adric out of his trance, they unknowingly awaken Zargo and Camilla. Tarak is literally throttled by Zargo and dies when his neck snaps on the edge of the "bed". Adric throws a knife at Zargo's heart and Camilla advances on Adric. Romana backs away but Zargo grabs her shoulder. With a toothy evil grin, he chuckles as he pulls the knife out of his heart and is about to give a fatal blow to Romana.
Romana closes her eyes as Zargo draws his arm back for a fatal blow but Aukon orders him to stop. "The boy," he says "is the first of the Chosen Ones, soon to be one of us. He is not for you!" Camilla then asks to have Romana. Aukon grabs Romana's chin and says gloatingly, "The girl is a Timelord, one of the enemies of the Great One. She is to be held for sacrifice at the time of Arising!" Romana and Adric argue about their fate. Adric says if it's a choice between death and joining the dine that means there's no sense in two of them getting the chop. Adric asks Aukon why he is being kept prisoner when Romana is the sacrifice and he is a Chosen One. "I'm sorry Timelady. One of my families died for your lot already. I reckon one's enough!" Romana asks Adric if he knows what happens to vampires when they die. Adric smugly replies "But they don't die, do they Aukon!". They are taken to the bottom of the Tower (as shown in the picture above), where Romana will be sacrificed at the time of Arising and Adric will indeed become a Chosen One.
In the TARDIS the Doctor and K-9 review the old stories about vampires. The lore that the Doctor and K-9 uncovers determines that the Great Vampires could only be defeated by metal bowships driven through their chests (rather like the wooden stakes that work on lesser vampires). He takes the craft to Kalmar's base and there uses scanning equipment to scan the Tower. Under the lake of blood he finds a restless, demonic presence, whom he determines to be the last Great Vampire. He warns it is about to be revived. Kalmar, Ivo and many other villagers agree to help him fight back. This rebel army and K9 make an assault on the Tower itself, killing collaborator guards. Ivo finds Habris and tells him "This is for my son!" and then kills him. The Doctor heads off to the peak of the Tower and, in a burst of characteristic ingenuity, the Doctor manages to rig one of the old scoutships still attached to the spaceship/tower to launch and achieve a trajectory that caused it to point back toward the ground and drive itself into the heart of the subterranean Great Vampire, thus duplicating the effect of the "metal bowships" of Time Lord history.
With the Great Vampire dispatched, the three vampire Lords crumble to dust without the power of their master to sustain them. The Doctor finds Romana and Adric. Together with K9 they leave the planet to its own fate, hoping that, now freed from the corruptive effect of the vampires, it will change direction and develop once again toward its former advanced state and even perhaps surpass it. He leaves the planet in Kalmar's hands, while hoping the next journeys in the TARDIS will take Adric home and lead the craft back out of E-Space.
Doctor Tom Baker (Fourth Doctor)
Companions John Leeson (K-9 Mk. II)
Lalla Ward (Romana II)
Matthew Waterhouse (Adric)
William Lindsay — Zargo
Rachel Davies — Camilla
Emrys James — Aukon
Clinton Greyn — Ivo
Rhoda Lewis — Marta
Dean Allen — Karl
Thane Bettany — Tarak
Iain Rattray — Habris
Arthur Hewlett — Kalmar
Stacy Davies — Veros
Stuart Fell — Roga
Stuart Blake — Zoldaz
Writer Terrance Dicks
Director Peter Moffatt
Script editor Christopher H. Bidmead
Producer John Nathan-Turner
Executive producer(s) Barry Letts
Production code 5P
Series Season 18
Length 4 episodes, 25 minutes each
Originally broadcast November 22–December 13, 1980