During World War II, two dinghies manned by Soviet soldiers row towards Maiden's Point on the English Northumbrian coast, but one gets lost in the fog. Nearby, the TARDIS materialises, and the Seventh Doctor and Ace step out. The Doctor is content to stroll right in, but Ace is wary — if this is a top-secret naval installation, then it should not be that easy. Indeed, only a few steps later they are confronted by armed guards. The Doctor befuddles the guards by complaining that it took them long enough and asks them where Dr Judson's office is. Going to see the wheelchair-using Judson, the Doctor ingratiates himself with him by recognizing the Prisoner's Dilemma which Judson has on a blackboard. Ace, too, impresses Judson by identifying a logic game that he also has mapped out. By the time the guard commander arrives to confront the Doctor and Ace about their unauthorised presence, the Doctor has forged a letter of authority.
The Soviets, meanwhile, have discovered the sole survivor of the other dinghy, delirious and pale. The Soviet commander, Captain Sorin, asks the survivor where the sealed orders are but he is unable to answer. The orders are discovered washed up on the shore by another Soviet soldier — inside is a photograph of Judson, but something in the water kills him before he can bring the orders back to Sorin. The Doctor and Ace have been put in barracks, and he tells her to go to bed while he walks around the grounds, noting to a guard that eyes are watching.
The next morning, the Reverend Wainwright finishes his service, speaking to a local spinster, Miss Hardaker, and her two charges, Jean and Phyllis, evacuees from the East End of London. The Doctor and Ace show up, looking for Judson, who is working on Viking inscriptions in the crypt of the church. Ace strikes up a conversation with Jean and Phyllis, and makes an appointment to meet up with them later at Maiden's Point. As they enter the crypt, Ace notices a lot of silver around and says they might get stolen. Wainwright assures her that the locals are too frightened of the Viking curse — the church was built on old Viking graves, and they say evil was once buried here. In the crypt, Judson is confident that the ULTIMA code machine, which he is using to decipher German intercepts, will be able to crack the inscriptions. Ace tells the Doctor she hears the sound of machinery, but the Doctor dismisses it as the sounds of the church organ.
The Doctor and Ace visit the churchyard, and note that it contains descendants of Vikings. When they go to Maiden's Point, they find the open packet of the sealed orders, and realize that there are Soviet soldiers about. The Doctor decides to go back to the church, and warns Ace not to go into the water. He finds Wainwright in the church, and correctly guesses that someone has already deciphered the inscriptions. Wainwright shows the Doctor a translation made by his grandfather, speaking of a curse following the Vikings who came here carrying Oriental treasure. They wished to go back to the "North Way" but the curse claimed their lives at Maiden's Bay. The Doctor reads part of the Soviet orders that mention Norway — the "North Way", and takes the translation to Judson.
The Doctor and Ace continue looking around the base, and come across the room where some WRNS are listening to coded transmissions. One of the WRNS, Kathleen Dudman, has a baby whom Ace takes to even though she is named Audrey, the same as Ace's mother, whom she hates. Millington, however, is not as sympathetic, and orders Kathleen to get Audrey off the base within 24 hours or face dismissal from the service. Exploring further, the Doctor and Ace enter Millington's office, a perfect replica of the German cipher room in Berlin, where he tries to divine the mind of the enemy. There are two differences, though — a picture that shows that Judson and Millington went to school together, and a Viking-themed chess set.
In his office, Judson continues to read the translation, which tells of the final battle of the gods at the end of the world, which Millington seems to believe is nigh. As Judson reads, new runes appear on the walls of the crypt in burning letters, and something stirs under the water. While searching Maiden's Point for more clues, Ace and the Doctor find the body of the soldier who found the orders earlier, with a strange object in his hand. They are then confronted at gunpoint by the other Soviets.
The soldiers take them back to Sorin once they discover that the Doctor and Ace know about their sealed orders. The Doctor tells Sorin that an assault on the base will be suicidal, and letting them go to investigate further is the only way to stop the evil that is killing his men. Already another soldier has been attacked and is delirious, holding on to a similar object which he hands to the Doctor.
In the meantime, Judson is carried away from deciphering the new inscriptions by his nurse, Crane, who finds the crypt too cold for an invalid. Millington insists that Judson use the ULTIMA to decode the inscriptions, even though it is needed for the German intercepts. The Doctor and Ace return to the crypt, noticing the new inscriptions. Remembering the machinery Ace heard earlier, they look for a secret door, but Millington finds them first, taking them at gunpoint into the hidden passages connected to the church. Inside, men are mining toxins from a natural source — a chemical weapon to end the war. The Doctor makes an allusion to the Well of Hvergelmir, where serpents spew their venom over the roots of Yggdrasil. This persuades Millington that the Doctor believes in Norse mythology as he does, and offers to show the Doctor all of it. The Doctor goes with Millington to the ULTIMA, leaving Ace behind in the church to talk to Wainwright, who seems to have lost his faith in humanity.
At the ULTIMA, Millington reveals that the computer is bait for the Soviets, with a beaker of toxin hidden inside the machine. Millington shows the Doctor a room with row on row of shells, and an isolation chamber to test the toxin. As a demonstration, he breaks a vial inside the chamber, killing a cage of pigeons. The plan is to let the Soviets take the ULTIMA, and when they try to decrypt the British codes, a single word — "Love" — implanted in a cipher will detonate the beaker.
In the crypt, a wall shakes open of its own accord, revealing an ancient looking urn, which is cast aside by the soldiers working there to seal up the church. At Maiden's Point, Jean and Phyllis enter the water again, fully dressed, splashing around as a mist drifts into the bay, and when it clears, they are gone. Millington is informed that the operations at the church are being closed down, and he orders that all radio transmitters and outside telephone lines be disabled and any chess sets in the camp be burnt. Judson starts to decrypt the inscription and the ULTIMA spits out, "Let the chains of Fenric shatter."
Jean and Phyllis rise from the waves, skins pale and with long claws in place of fingernails. They lure a Soviet soldier into the water, but before he can reach them, monstrous hands grab and drag him under. Ace realizes what the inscriptions are — a complex logic diagram for a computer program, and tells Judson this in his office. Armed with this knowledge, Judson excitedly tells Crane to take him to the ULTIMA room. Meanwhile, Jean and Phyllis go to Miss Hardaker and kill her. When Ace and the Doctor find Hardaker's body drained of blood, the two vampire girls are already approaching Wainwright, who lost his faith when he heard about British bombs killing innocents. They are about to feed on him, but the Doctor arrives and dismisses them. They threaten to return for Wainwright.
The Doctor says that as long as Judson does not solve the inscriptions, everything will be all right. Ace, horrified, realizes she has given Judson the solution and says the Doctor should have told her. They rush to the ULTIMA room as more monstrous vampires, some dressed in archaic clothing, rise from the bay. When the Doctor, Ace and Wainwright reaches the machine, it is running at top speed and cannot be stopped. The urn in the crypt glows with a green light, as Millington says triumphantly that the Doctor is too late.
Millington is convinced that once the chains of Fenric shatter, all its dark powers will be his. The Doctor points out that the base's defences have been weakened because Millington wanted the Soviets to steal the machine, and reinforcements cannot be called for because the transmitters have been disabled. The Doctor explains that the vampires are really Haemovores, mutations from mankind's far future. Ace wants to make sure that Kathleen and Audrey are all right before they go back to the church. Wainwright traces the names of the descendants of the Viking settlers while the Doctor and Ace explore the crypt for clues. Ace finds the cast-aside urn, and puts it in her backpack without telling the Doctor, not considering it the Oriental treasure he is looking for. Wainwright finds the names the Doctor is looking for — the curse of Fenric passing down through the generations.
The Haemovores attack the church, Wainwright and Ace trying to beat them back with the church silver. Ace tries to escape through the roof, but is attacked by the haemovores. Sorin and two of his men spot her struggles and rescue her, although their shots only slow the haemovores down. What eventually drives them back however, is the Doctor's faith — as he chants the names of his former companions, the haemovores scream and retreat. The Doctor explains that faith forms a psychic barrier that the haemovores cannot penetrate. The Doctor orders everyone into the crypt, but Sorin says he needs to go back to his men. His faith in The Revolution will protect him. He gives Ace his scarf, and tells her to be careful.
The Doctor finds the tunnel to the toxin stores (and a back way out) sealed, but Ace blows it open with two cans of Nitro-9. The two Soviet soldiers stay a bit behind to slow the haemovores down. Ace takes out the flask she found, hoping to use it to mix more Nitro-9, but the Doctor recognizes it as the treasure they've been searching for. Outside, Sorin keeps the haemovores at bay with his Soviet Army badge, but they turn away from him and start moving back to the church, being drawn to the flask. When the Doctor, Wainwright and Ace reach the exit, Millington is waiting, and takes the flask, sealing the exit even though the two Soviet soldiers are still inside, now trapped with the haemovores. They plead with Millington to open the gates, but their pleas are ignored.
Sorin moves back into the camp, telling his men to abort the mission if he is not back in ten minutes. He calls out to Millington to talk terms, officer to officer. However, Millington is not interested, and places Sorin under arrest. Sorin manages to warn his men, and they retreat. The haemovores start to burn through the metal shutters of the tunnel, while Ace discovers Kathleen sitting alone in the barracks, numb with grief at receiving a letter informing her of her merchant navy husband's death at sea. Judson continues with the decoding of the inscription, now with the flask sitting in the ULTIMA.
Ace confronts the Doctor, demanding to know what he knows, bitterly complaining that he always seems to know, but just cannot be bothered to tell anyone, like it is a game that only he knows the rules to. The Doctor tells Ace about an ancient evil that has existed since the birth of the universe. Fenric is just Millington's name for it, and it is trapped in the flask. Ace lures Sorin's guard away while the Doctor frees the Soviet captain — they have to stop Fenric before he finds a body. Wainwright waits while the haemovores break through the tunnel gates, and tries to hold them back with the Bible, but his faith is too weak and the haemovores overwhelm him.
The flask in the ULTIMA flashes, hitting Judson with green energy, causing him to collapse out of his wheelchair. A storm whips up as the haemovores advance over Wainwright's lifeless body. As the Doctor, Ace and Sorin reach the ULTIMA room, Millington murmurs that the chains of Fenric are shattered and the gods have lost the final battle. Ace thinks that Fenric has taken Millington, but she is wrong. Behind the Doctor, Judson rises to his feet, his eyes aglow, and proclaiming to the Time Lord that the contest is to be played anew.
Fenric says that the Doctor trapped him in the Shadow Dimensions for seventeen centuries, but now he has a body again and the preparations are complete. He teleports away as British soldiers rush in. Millington orders that the Doctor, Ace and Sorin be shot for treason. Before the firing squad can carry the order out, however, Sorin's men attack and the three escape. Fenric appears in the tunnels, meeting the haemovores and asking them to fetch him the Ancient One. At the bay, the haemovores perform the summoning, and the Ancient One, a hideously mutated haemovore, rises from the waters. The Ancient One is from the far future, a time where it is the last thing left alive, the ultimate destiny that evolution and pollution has shaped for mankind.
The Doctor tells Ace that he needs a chess set, and they remember the one in Millington's office, but Millington has wired the chess set to explode, and they barely get out before the office is destroyed. Millington, meanwhile, meets Fenric in the toxin chamber, where Fenric reminisces about the time the Doctor beat him — carving pieces out of bones taken from the desert sand and setting up a chess problem. Fenric was unable to solve the puzzle and was banished, so now he seeks revenge. The Doctor explains to Ace why the names are important - they tell of where the curse has passed, to family names like Judson, Millington, Wainwright and Dudman. Ace remembers that Kathleen still has a chess set which escaped Millington's destruction order, and they go to retrieve it.
The British soldiers use toxin grenades against the Soviets, but the haemovores start killing the British soldiers. The WRNS become haemovores and turn on the soldiers who come to rescue them. Fenric allows the haemovores to kill Nurse Crane, in return for all the humiliation she visited on Judson over the years. When Bates, the guard commander, goes to warn Millington of the massacre, he overhears Fenric tell Millington that he will use the toxin stores to poison the world forever. Bates sneaks out again, and joins forces with the Doctor and Sorin to fight the real enemy. Sorin praises Ace for her bravery, and gives her his Soviet Army badge. Ace and the Doctor reach the barracks and take Kathleen's chess set. Ace stays to look after Kathleen and Audrey. The haemovores crash in through a window, and they barely escape out another one. Ace bundles Kathleen and Audrey into a jeep, telling them to go to London, where Ace's nan will look after them. Kathleen gives Ace a picture of Audrey, and Ace kisses the baby goodbye, saying she will always love her.
Fenric tells the Ancient One to take the poison into the oceans, and kill the other haemovores. He concentrates, and the haemovores collapse and crumble to dust. The Doctor sets up the chess puzzle in the toxin stores, and challenges Fenric to solve it. Millington is about to shoot Bates but Sorin's last man shoots Millington instead. As Fenric desperately contemplates the puzzle, the Doctor speaks to the Ancient One. He knows the Ancient One comes from a world where industrial pollution has caused the Earth to die in a chemical slime, and what Fenric is asking the haemovore to do is to be the act that begins that — another one of Fenric's games.
When Sorin goes to kill Fenric, the weakened entity possessing Judson tells Sorin that he was chosen for this mission because his grandmother was English, and a descendant of one of the Viking settlers. Sorin is touched by the curse, too — one of the Wolves of Fenric. When Ace enters the toxin store, it is Sorin who is contemplating the puzzle, and Ace tells him the solution, realizing too late that Fenric is now in Sorin's body. Fenric solves the puzzle as the Doctor enters, and he gloats. All the descendants of the Viking who buried the flask were pawns in his game - Judson, Millington, Sorin, the Ancient One, and even Ace. In thirty years, Kathleen Dudman's daughter Audrey will become Ace's mother, the one Ace hates. In saving Kathleen and Audrey, Ace has created her own future.
Fenric orders the Ancient One to kill them. Ace, however, still has faith in the Doctor's infallibility, and that faith creates a psychic barrier that keeps the Ancient One back. Fenric takes up a vial of the toxin, saying that he will kill the Doctor anyway, but if he would like Ace to live, to kneel before him. The Doctor callously tells Fenric to kill Ace, revealing that he knew all along that Ace had Fenric's evil inside her. Why else would he have taken in a social misfit and an emotional cripple who could not even pass chemistry, yet somehow created a time storm in her bedroom? The Doctor saw Fenric's hand in it from the start (as soon as he saw the chess set in Lady Peinforte's study), and was just using Ace to get to him. With these revelations, Ace's faith in the Doctor is shattered, and she falls to the floor weeping. The Ancient One is free to move, but instead of killing the Doctor and Ace, forces Fenric into the isolation chamber, shattering the vial of poison and killing them both. The Doctor pulls Ace out of the store as it explodes behind them.
The Doctor tries to explain to Ace that he did not mean what he said, but had to break Ace's faith in him to allow the Ancient One to act. He would have done anything not to hurt Ace, but he had to save her from the curse. On the shores of Maiden's Point, Ace wonders why she cannot stop hating Audrey, her mother, even though she loved her as a baby. Ace dives into the bay, no longer frightened of the water, and surfaces, liberated. The Doctor and Ace walk off, back to the TARDIS.
Doctor Sylvester McCoy (Seventh Doctor)
Companion Sophie Aldred (Ace)
Dinsdale Landen — Dr Judson
Alfred Lynch — Commander Millington
Stevan Rimkus — Captain Bates
Marcus Hutton — Sgt Leigh
Christien Anholt — Perkins
Tomek Bork — Captain Sorin
Peter Czajkowski — Sgt Prozorov
Marek Anton — Vershinin
Mark Conrad — Petrossian
Nicholas Parsons — Rev Wainwright
Janet Henfrey — Miss Hardaker
Joann Kenny — Jean
Joanne Bell — Phyllis
Anne Reid — Nurse Crane
Cory Pulman — Kathleen Dudman
Aaron Hanley — Baby Audrey
Raymond Trickett — Ancient Haemovore
Writer Ian Briggs
Director Nicholas Mallett
John Nathan-Turner (uncredited)
Script editor Andrew Cartmel
Producer John Nathan-Turner
Executive producer(s) None
Production code 7M
Series Season 26
Length 4 episodes, 25 minutes each
Originally broadcast October 25–November 15, 1989