The Time Lady Ruath, a scholar of ancient Gallifreyan history, has become convinced that at the end of his life Rassilon himself had become a vampire -- and that it is the Time Lords’ destiny to become a vampiric race. She has participated in blood rituals with the cults that worship Rassilon the Vampire, and through them she has acquired the Ring of Rassilon, both the symbol of the Great Vamipre’s defeat and a means of remotely controlling all Time Lord technology. Following the Agonal incident and the Seventh Doctor’s departure from Gallifrey, Ruath realises too late that the events which have just come to pass were predicted by the ancient Time Lords -- and that the vampire lord Yarven has just been taken to Earth on the Doctor’s TARDIS. Ruath therefore captures Romana and forces her to take Ruath into the Presidential chambers, where Ruath shoots Flavia with a stunner and uses the Ring of Rassilon to access the Timescoop. When Romana tries to intervene, Ruath sends her to a swamp full of Drashigs -- in fact an environment in a MiniScope now owned by the interstellar rogues Glitz and Dibber. Ruath then uses the Timescoop to locate the Doctor. Rejecting his devious seventh incarnation, and his astonishing final form, she eventually chooses to attack the more vulnerable Fifth Doctor…

Ruath destroys the Timescoop chamber behind her, steals a TARDIS and travels to the city of Manchester in 1994. There, she contacts goth vampire lovers Jake and Madeleine, and local Manchester vampires Eric and Jeremy. Using the ring to convince them that she heralds the return of their Messiah, she takes them to Croatia, where Yarven was buried alive in the 1940s after encountering soldiers armed with faith and crucifixes. Ruath and the vampires exhume him, but fifty years under the soil have left him emaciated. Before he can take his rightful place as their ruler he needs some very special blood -- and Ruath knows just where to find it.

The Doctor and his companions are in Tasmania, where the Doctor is participating in a charity cricket match while Tegan, still recovering from the trauma of her possession by the Mara, is reading the works of Primo Levi. As night falls, Ruath sends a baby vampire, the Child, to leech out the Doctor’s blood, but it errs and attacks Nyssa instead. Tegan hears the commotion and interrupts before the Child can drain all of Nyssa’s blood -- but Nyssa finds herself literally incapable of telling her friends what has just happened to her. Meanwhile, having failed to obtain the Doctor’s blood, Ruath sacrifices herself instead, using her TARDIS to drain out every drop of her own blood and filter it through to Yarven. She then regenerates, and Yarven, restored to full health -- and given the symbiotic nuclei of a Time Lord -- rewards her by biting her and making her his vampire consort. Now the age of the Undead shall begin.

Nyssa finds sunlight an irritation, and is unable to reach the TARDIS. She hopes to find a cure for her condition, but her transformation has given the vampires a psychic link to her, which they are using to prevent her from telling the Doctor what has happened. That night they attack again, but Tegan uses her faith in Primo Levi’s works to drive off Jeremy, and the Doctor traps Eric in a circle of garlic bread. The Doctor realises that Eric is from Manchester, but before he can learn anything more, Yarven telepathically forces Eric to kill himself and tells the others that the Doctor was responsible. The Doctor, concerned by the thought of vampires working together, takes Tegan and Nyssa to Manchester to investigate further, but Ruath gets there first by using her TARDIS to travel back in Time. By the time the Doctor arrives, the vampires have been hunting for three weeks, and the city is starting to panic as homeless people, students and others walking alone at night begin to vanish…

Nyssa and Tegan set out into the city to investigate further, but although it is overcast Nyssa is still panic-stricken by the thought of the sun coming out and flees back to the TARDIS. There, she tries to create a synthetic haemoglobin substitute, but fails. Unable to control her thirst, she attacks Tegan, but at the last moment she manages to resist her urge to feed, and flees from the TARDIS into the night. While feeding on a cow in the fields outside the city, she meets Jeremy, who takes her to the vampires’ new home -- Castle Yarven, a gothic fortress which has appeared in the forest outside Manchester. Yarven welcomes her to his army, but assures her that if she does not wish to remain a vampire, then Ruath will see to it that her condition is cured by the next full moon. Relieved, Nyssa agrees to remain in the castle -- unaware that Yarven has no intention of keeping his word.

The Doctor and Tegan go vampire hunting, and capture Jake and Madeleine, but before they can learn where Nyssa is, they are interrupted by American evangelist Victor Lang and a gang of local Christians. The Doctor releases the vampires before the Christians arrive, telling them to ensure that Nyssa remains safe. He is somewhat put off by Lang’s claims that this city has become a hotbed for Satanic cultists who torture their own children in their ceremonies, feeling that this sensationalist approach distracts attention from the real causes of child abuse. However, Lang claims to have been contacted by a frightened young woman who knows that a Black Sabbat is soon to be held nearby. The Doctor decides to accompany him on the raid to find out what’s really going on, and on the way, he slips the Christians garlic capsules disguised as chocolate treats, hoping to arm them against the vampires without having to explain what he expects them to encounter. As he had expected, the “cultists” are vampires and the Sabbat is a trap, but even the Doctor isn’t prepared for the weapons which the vampires use against their attackers -- mortar shells containing a mist that causes most of the Christians to spontaneously combust, apart from one young boy named Matthew who claims that he is now damned forever. Madeleine pulls Lang out of the carnage and takes him to Castle Yarven, where he is placed in a pit while he sweats the garlic out of his system.

The Doctor and Tegan take the terrified Matthew home and then return to the TARDIS, where the Doctor studies tissue samples from the immolated Christians. Tegan, unwilling to do nothing while the Doctor works, leaves the TARDIS to see if she can learn anything more, and hears news of an atrocity at a familiar address. Matthew has turned into a vampire overnight and is hiding in the sewers, having killed his own parents. The Doctor, meanwhile, distils an odd chemical out of the tissue samples, and takes it outside to expose it to the sunlight -- but the sun abruptly sets two hours ahead of schedule, and the Doctor collapses in agony as his time senses overload. As he falls, the chemical bursts free of its container and attacks him like a living thing. While he is unconscious, Romana -- who has been rescued by the Time Lords and taken back to Gallifrey -- contacts him telepathically. She is not permitted to tell him too much about his future, but she does give him one vital piece of information…

Matthew emerges from the sewers and attacks Tegan, but the daylight suddenly returns, burning him and forcing him to return to the sewers. Ruath’s prototype Time Freeze has short-circuited; she had been testing it with the brain of a goat, but it seems to require a more sophisticated life form to keep going. According to prophecy, this is to be Yarven, who will give his life for his people; however, Yarven has his own ideas… Meanwhile, Lang’s system is now free of garlic, and he is therefore gassed, removed from the pit and operated upon. Nyssa gets in to watch the operation, but as she rests her hands on Lang’s forehead, Ruath casually informs her that Yarven lied about the cure; there will never be another full moon, and soon everyone on Earth will be a vampire. Nyssa tries to flee, but Ruath, knowing that Nyssa has been given enough time to send a message to the Doctor and lure him into their trap, catches her and buries her alive.

The plasma which was attacking the Doctor burns to ashes in the sunlight, and further analysis confirms the Doctor’s suspicions; the vampires have developed a vaporised form of vampire DNA, which can be absorbed through the skin. Anyone exposed to the mist will be transformed into a vampire, except for those who are protected by faith or garlic -- they will spontaneously combust. The vampires release Lang, intending to use him to demonstrate their power, but as Madeline returns him to Manchester she reads his half-delirious mind and realises what truly lies behind his belief in redemption. Before dropping him off, she plants a post-hypnotic command ordering him to reveal the truth. Lang recovers to find himself back in Manchester, and convinces himself that somehow God has saved him from the demons in the castle. The people of the city, terrified by the previous day’s premature night, flock to hear his words, and when the Doctor and Tegan encounter the crowd they see numbers appearing on Lang’s forehead -- the map co-ordinates of Castle Yarven, which Nyssa wrote on his skin with heat-sensitive chemicals while observing the operation.

The Doctor and Tegan break into Castle Yarven, which turns out to be Ruath’s TARDIS. There, they split up to search for Nyssa, and Tegan finds her first -- but hours trapped under the soil have driven Nyssa mad with thirst. Meanwhile, the Doctor finds the sleeping Yarven and reluctantly prepares to stake him -- but when he places the stake over Yarven’s heart this triggers a trip circuit which activates the Time Freeze, now linked to the volunteer Jeremy’s brain. Night falls over Manchester at two in the afternoon. Ruath saves Tegan from the animalistic Nyssa, and throws her and the Doctor into the pit, explaining that while the Time Freeze holds the western hemisphere in perpetual night, Yarven’s vampire army will release their vapour weapon into the winds. Soon the entire population of Earth will become vampires, and through sheer force of numbers they will conquer Gallifrey.

The terrified citizens of Manchester, fearing that the end of the world is coming, flock to Old Trafford stadium to hear Lang speak. Lang steps forward to address the masses -- and then Madeleine’s implanted command takes effect, and he unexpectedly blurts out that he’s had an incestuous relationship with his daughter. As his stunned congregation watches, the genetic material the vampires planted in his body is released, and Lang transforms into a vampire before their eyes. The missing children of Manchester then return to them, releasing the vampire vapour into the stadium, and as the Child descends from the skies to join Lang on stage, the entire congregation begins to transform or burn…

Tegan learns that Ruath is an old friend of the Doctor’s, and that at one point the Doctor apparently intended to flee Gallifrey with her. Ruath explains her belief, based on ancient texts, that the Time Lords are destined to become a vampire race, and asks the Doctor to become her consort while Yarven is sacrificed to the Time Freeze. The Doctor allows Ruath to bite him, at least partly to keep her away from Tegan. But as Ruath prepares to install Yarven in the Time Freeze, Yarven takes her prisoner, revealing that Agonal appeared to him once more on his home world to warn him that his future consort would betray him. He has known all along that Ruath only intended to use his kind as breeding stock to strengthen the Time Lord race, and he no longer requires her services.

Yarven prepares to install the Doctor in the Time Freeze machinery, but at the last moment, the Doctor, alerted earlier by Romana, shouts out a command to Ruath’s ring. The Ring of Rassilon controls all Time Lord technology, including each and every TARDIS, and at the Doctor’s command, Castle Yarven dematerialises -- and the Time Freeze is thus broken. Daylight returns to Manchester, and the vampire army, caught off guard, bursts into flame. Lang, Matthew and the Child are all immolated, and with the Child’s death, Nyssa -- who had never fed first-hand on human blood -- is restored to normal. In the confusion, the Doctor, Tegan and Nyssa escape, and emerge from Ruath’s TARDIS just as it materialises on a distant planet which the Doctor has visited before. The chameleon circuit transforms Ruath’s TARDIS into a flat disc, open to the elements -- but the sun has just set. The Doctor takes the opportunity to offer Yarven a chance to settle on this world, where his people can develop an artificial food source and live without needing to kill. Yarven rejects this offer, and prepares to kill him -- but before he can do so, the planet’s second sun rises, as the Doctor had known it would. Caught entirely off guard, Yarven and his vampires are immolated, while the Doctor, who has only been a vampire for a few minutes, survives his exposure to the sunlight.

The Doctor pilots the few survivors back to Earth, and programmes Ruath’s stolen TARDIS to return to Gallifrey. He then returns to his own TARDIS with Nyssa and Tegan, only to find Ruath waiting; she used her ship’s chameleon circuit to transfer herself into the Doctor’s TARDIS without ever leaving the timeless environment inside, and is thus still a vampire. She accuses the Doctor of abandoning her on Gallifrey, but he reveals that he did so deliberately, knowing that she wouldn’t be able to handle life as a renegade. He thought she would do better as a political rebel fighting the system from within -- but he didn’t expect her love of the past to develop into insanity. Furious, she attacks him, but Nyssa opens the TARDIS doors in mid-flight and expels Ruath into the Vortex. The Doctor is thus cured of his own vampirism.

The Doctor, Tegan and Nyssa return to Tasmania for the end of the charity cricket match, and then depart for adventures elsewhere. Back on Gallifrey, Romana is offered a position on the High Council for her services. And on the outskirts of Earth’s solar system, Jake and Madeleine -- who had given up on Ruath some time ago -- strap themselves and two canisters of vapour to the departing Voyager II probe, and settle in for a long sleep to see what the future holds.