Darkness has fallen as Habris, Captain of the Guard, seeks audience with his King and Queen. Standing fearfully before the sinister monarchs Habris proclaims it is once more the time of selection. King Zargo advises Habris to choose well - those selected should be young, strong and full of life. Standing to one side Aukon, the Councillor, reminds Zargo that it is spirit the Great One will prize most. He still seeks in vain for the first of the Chosen Ones who will serve at the Time of Arising. Gratefully Habris is allowed to take his leave.

Next morning, in a nearby village, Ivo headman of the community, has summoned all the adolescent young for selection by Habris. Grimly the Guard Captain advances down the line tapping on the shoulder those he considers suitable. To the dismay of Marta, Ivo's wife, Habris picks out Karl, her son who has not been included in the line-up. Ivo is angry. Habris replies there is no choice in the matter. Despite their mutual respect for one another Habris knows he could not keep back any secrets from Aukon. Somehow the Lords can see into their minds. If Karl is lucky he will be allowed to become a guard. If not then he will go to serve the Lords directly. And no-one who has done that, points out Ivo, has ever been heard from again...

Aboard the TARDIS the Doctor and Romana are puzzling their latest quandry. Thanks to their passing through a Charged Vacuum Emboitment recently they are now trapped in the greenish void of E-Space, a smaller Universe quite separate from normal N-Space. Unable to navigate properly they are consequently lost and Romana is doubtful they can find an inhabited world in this desolation. To their surprise K-9 locates one although scanners reveal there is only a single, localised concentration of civilisation. Feeling he might be able to enlist the help of any indigenous scientists there the Doctor decides to land.

The TARDIS comes to rest in a forest and, with the aid of a telescope, the Doctor espies the medieval-type community they had detected from space. Leaving K-9 behind to resolve the question of navigating by negative co-ordinates, the two time travellers set off to explore.

K-9's computations are interrupted by the entrance of an unexpected visitor. This is Adric, a boy they had met on the planet Alzarius. Having hidden aboard as a stowaway Adric is now keen to make his presence known and he leaves to follow the Doctor and Romana.

Entering in the village the two time travellers find their way to the main community hall. Here they disturb a debate between Ivo and Habris. The Doctor is unable to get any sense from the incredulous Ivo who reacts almost with alarm to the Doctor's request to meet any scientists and suggests the Doctor looks elsewhere for his answers. As the pair leave Ivo opens a cupboard and takes out a rusting transceiver set. Contacting a person named Kalmar he relates the news of the strangers and, in particular, their wish to meet scientists.

In the throne room of the Tower Habris is being admonished by his monarchs for not arresting the Doctor and Romana. They must be found at once. Aukon, however, says there is an easier way than using the guards - if they are on this planet his servants will find them. Concentrating, Aukon's mind's eye focusses on the image of a flying bat.

Returning through the forest Romana and the Doctor are surrounded by a group of cowelled men and led to a concealed cave where they meet Kalmar. Kalmar is trying to get an old monitor screen working, and he is more than pleased when the Doctor helps him repair part of it. Romana identifies the equipment as a data bank of Earth origin, from the cargo vessel Hydrax. Images flash up ot the three officers, Captain Sharksy, Lauren MacMillan and Anthony O'Connor. Tarak, Kalmar's deputy, points out the faces are familiar-they are the faces of the Three Who Rule. The Doctor would like a meeting with them and, with Kalmar's permission, they are allowed to leave.

With night falling Adric steals into the village Hall and attempts to purloin some food. He is caught by Marta and Ivo, but they take pity on the boy and offer him sanctuary. He cannot go out now; it is' nearly dark and evil stalks the night on this world...

At the last moment the bats change course and fly off. But as Romana and the Doctor struggle to their feet they are met by Habris and his men who have come to "escort" them to an audience at the Tower.

They are shown into the huge, circular throne room, the Doctor remarking it strange there appear to be no windows in this edifice. Almost from no-where Zargo and Camilla appear, bidding the two travellers welcome. Zargo suggests some refreshments for his guests, which include wine and meat. Zargo offers a toast to his guests. The Doctor clinks glasses with Romana but does it too hard causing Romana’s glass to shatter to pieces in her hand. The broken pieces end up on the floor. Romana licks her finger, which has been cut by the glass. This tempts Camilla, who wants to look at the cut. However, Romana pulls her hand away, saying it’s only a cut. As discussions unfold it becomes increasingly apparent to the Doctor the lack of advance on this planet is due entirely to the influence of these rulers. Camilla agrees they have lost their technology over the years, but adds there are compensations...

In the Village Adric is helping with the evening meal when Habris enters. There is to be another selection now. Ivo moves to protest but is told Lord Aukon himself is here. Aukon walks down the line of youths, stopping when he comes to Adric. He recognises the first of the Chosen Ones. Trance-like Adric follows Aukon from the hall.

Mention of the Hydrax disturbs Zargo and Camilla and the Doctor is about to pursue this when Habris enters with news from Aukon that the Time of Arising is at hand. Being left alone gives the Doctor a chance to look around. He pulls aside a drape to reveal a circular hatchway in a metal bulkhead. This Tower is, or was, the Hydrax! The Doctor decides it is time to explore further.

News of the Doctor's capture has prompted some of the outlaws to suggest a rescue attempt. He is their first real hope in a thousand years. But Kalmar is reluctant without more knowledge on their side, and the majority back him up. Disgusted, Tarak says he will go alone.

At the top of the Tower Romana is amazed to find herself in the cramped flight deck of a small one-man space ship. The Doctor identifies it as an Arrow Class scout ship, one of three aboard the Hydrax. Despite their age there is still a little power in the batteries. Romana hears a dull thumping noise coming from deep in the main ship, which the Doctor suggests, grimly, is more evidence. They begin the long climb down.

Possessed with fanatical zeal Aukon suggests the Doctor should join them. They seek people with powerful minds to help at the Time of Arising. For only then will the Great One, who brought the Hydrax into this Universe reveal the secret of how to get back. The Great One speaks through Aukon and he, like Camilla and Zargo, feed and nourish the creature in return. If the Doctor is wise he will join them, just like his other companion. Perplexed at first by this remark, Romana and the Doctor are gravely worried to learn that Adric is here, and, even worse, in the power of these ghouls. Aukon uses all his psychic powers to make the Doctor bend to his will. Romana picks up a piece of the cave wall in a stake like form. She throws it at Aukon, who quickly turns around and destroys it. The Doctor comments his powers will not work on Time Lords. This astonishes Aukon. The Doctor and Romana try to escape but Zargo and Camilla block their paths. Camilla is eager to feed on the two Time Lords. Suddenly, the councillor tells Zargo and Camilla that the Great One is calling them. Camilla and Zargo stand still while Aukon goes to the altar, kneels down and asks for the Great One for his command. Aukon says to the Great One that tonight he’ll feed on the blood of their ancient enemies. After the command is given, Aukon tells the Doctor and Romana they will be used as sacrifices for the Time of Arising.

Nearly daybreak. Tarak has gained entry to the Tower, "borrowing" the uniform of a guard along the way. Watching, out of sight he sees Zargo and Camilla enter their locked Inner Sanctum, and also the imprisoning of the Doctor and Romana in a nearby cell, similarly locked and guarded.

To pass the time the Doctor recounts a story told to him on Gallifrey concerning a race of Giant Vampires. They swarmed all over the Universe, but the Time Lords hunted down the creatures in a war so long and bloody that it sickened them ot violence forever. However, when it came to counting bodies, the King Vampire was found to be missing. They followed a trail of his blood right across the Universe, but it came to a dead end: just vanished without trace.

Overpowering the guards Tarak rescues the Doctor and Romana, but Romana refuses to leave until she has found Adric. The Doctor asks Tarak to stay with her while he goes back to the TARDIS.

At the rebel headquarters Kalmar has discovered a new facility on his monitor system - a method of converting it into a scanner for viewing the territory surrounding their base. It picks up a figure which turns out to be Ivo. Embittered Ivo relates he now knows his son to be dead - he has come here to raise supporters for an attack on the Tower tonight. Thanks to a drunken guard Ivo knows the Three will be busy at some kind of ceremony, so it is now or never for him and his men. But still Kalmar is undecided...

“No!” comes a voice. It is Aukon. Zargo tells Aukon he is too late to interfere. The councillor says that Adric is to be the Chosen One. Camilla asks if they can have Romana. However, Aukon has a better idea. Romana is a Time Lord and she can be used as an ideal sacrifice to the Great One tonight.

When the Great One emerges the villagers will perform one last service as nourishment for the King Vampire. Then all four of them will leave for the universe outside, sucking the blood of rich worlds until they are but empty husks. Romana and Adric are led away as preparations begin.

Inside the TARDIS the Doctor realises he needs help from the rebels to stand a chance of rescuing his companions and defeating the vampires. In a bid to win them over he makes an impressive entrance - materialising the TARDIS in the middle of their base. Ivo and his men stand ready, but still Kalmar is reluctant to help without further time needed to gain knowledge. The Doctor tells them they only have a few hours before the creature wakes and destroys them all. He uses Kalmar's monitor to show the rebels an image of the King Vampire lying asleep beneath the Tower. That, coupled with the sound of the creature's heart beating, sways the final doubters.

Fully awake, Adric has been catching up with news of events from Romana. Reviewing the evidence he tells her he believes this is one time the good guys won't win. Still, he says, he's all right, he's being offered a partnership, so there's no sense both of them getting the chop. Incredulous, Romana reminds him the Doctor has spent all his life coping with threats like this, and he will try to save them. Ignoring this Adric summons Aukon and tells him he will join the Three of his own free will. He is released.

With quite a force behind him now, the Doctor's main worry is how to kill the Great Vampire. Their cardio-vascular structure is such that they can just seal off any minor wounds. What is needed is an arrow of steel. Suddenly realisation dawns. He tells the rebels to organise a commando force and loans them K-9 whose weaponry will be invaluable in the attack. Whatever happens they are to hold the Tower until they get a signal from K-9 to make their way round to the caves and try to rescue Romana and Adric. He, meantime, will be occupied elsewhere...

The raid begins. With K-9 in the lead, disposing of those guards armed with blasters, rebels overpower the remaining, more basically armed soldiers.

Hypnotised Romana is led to the sacrificial altar. The air above the cavern blackens as the bats gather in full swarm to take bites from Romana's neck.

Up above the attack is going well. Ivo finds Habris. Habris says to Ivo he tried to help Karl but he rebelled and refused to serve the Lords and was killed by Zargo. Habris goes on to say that Zargo is in the caves and tells Ivo he will take him to him. However, Ivo says he’ll find Zargo for himself and strangles Habris in vengeance for his son’s murder. The Doctor disappears up to the Tower summit and, on his third attempt, finds one of the scout craft with enough fuel and power for one last, but vital, journey on auto-pilot. Frantically he programs in the flight coordinates, and then leaves as the take-off sequence begins.

Hearing the engines start up K-9 advises the rebels to evacuate immediately. Down in the caves the low vibration partially disrupts the ceremony, giving Adric enough time to drag Romana from the Altar. The Three are exultant to see the ground before them splitting, revealing a claw-like hand of the Great Vampire. The Time of Arising is now. Huddled in a small niche in the cave walls, Romana and Adric are joined by the Doctor as they watch the grim finale.

Blasting free of the Tower the Arrow ship rises up until its retros fire, turning it through 180 degrees. Now, like a javelin it plummets back towards the ground and impales the King Vampire through the heart like a giant stake. As the creature dies the Three, in a frenzy, turn on the Doctor determined to take every drop of his blood. But their time too is up. Robbed of the Vampire's power, Zargo, Camilla and Aukon wither in seconds to dust.