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Russian commandos (Sergeant Prozorov: Peter Czajkowski; Petrossian: Mark Conrad; others: Andy Combs, Ken Dee, Martin Dew, David Foster, Damon Jeffrey, Nigel Parkes Davies, Mark Ponsford, Vince Sears, Derek Van Weenan) arrive on the English coast by dinghy. Their commander, Sorin (Tomek Bork), orders them to set up camp. They find one of their comrades lying injured on the rocky beach and Sorin tries to discover what happened to the sealed orders that he was carrying.
The TARDIS arrives just outside a secret naval base. The Doctor leads Ace to the office of the wheelchair-bound base scientist Dr Judson. There he forges letters of authority for himself, completing this task just before Captain Bates (Steven Rimkus) arrives to investigate. The two travellers are then shown to their quarters for the night.
One of the Russian soldiers, Gayev (Stephen Fitzalan), finds the sealed orders but is chased and attacked by something unknown.
The next morning, the Doctor and Ace arrive at the local church looking for Judson. The vicar, Mr Wainwright (Nicholas Parsons), shows the Doctor to the crypt. Ace meanwhile befriends two young women, Jean (Joann Kenny) and Phyllis (Joanne Bell), evacuees from London who are currently staying with an elderly woman named Miss Hardaker (Janet Henfrey). They arrange to meet later at Maidens’ Point, then Ace rejoins the Doctor. In the crypt, Judson (Dinsdale Landen) is trying to translate some Viking runes. The Doctor and Ace leave him to it. After a detour through the churchyard, where the Doctor realises that some of the graves have Viking names on them, the two travellers head out to Maidens’ Point. There the Doctor finds the sealed orders. He returns to the church, where Wainwright shows him some records made by his grandfather at the end of the last century. They contain translations of the inscriptions in the crypt. The Doctor and Wainwright take the translations to Judson.
Back at the base, the Doctor and Ace find a group of Wrens (Marianne Bergin, Mandy Demetrious, Claudia Lyster, Nicola Maddock, Suzi Mollett, Jane Perry, Roslyn Riley, Kate Shury, Wendy Spear) translating German messages. One of the women, Kathleen Dudman (Cory Pulman), has a baby girl (Aaron Hanley) with her, and Ace falls in love with it.
The Doctor and Ace find Commander Millington’s office, which is an exact replica of a naval cipher room in Berlin. Millington (Alfred Lynch) is trying to think like the enemy in order to stay one step ahead.
The Doctor and Ace return to Maidens’ Point and find a dead soldier. They are captured by Sorin and his men but, on hearing the Doctor’s story, Sorin lets them go.
Judson shows Millington the translations. As he reads the words, things move under the sea and a set of new runes appears in the church crypt. Later, Millington suggests that Judson use the Ultima machine – a code breaker – to translate the runes.
The Doctor and Ace find the new inscriptions in the crypt but are interrupted by Millington. He takes them through to a secret tunnel system where a natural poison that seeps from the walls is being collected. Millington plans to use the poison to win the war.
The Doctor goes with Millington back to the base as Ace speaks with Wainwright. The minister is suffering a crisis of faith.
Millington explains to the Doctor that the Ultima machine has been booby trapped with a flask of the poison. The plan is that the Russians should be allowed to steal it. He demonstrates the effects of the poison in an isolation tank. The machine is programmed to detonate the flask when it receives a particular coded word: ‘Love’. That word will be included in a message sent to Russia when the political climate is right.
Miss Hardacre has severely reprimanded Jean and Phyllis for going to Maidens’ Point. They ignore her and return there, venturing into the water for a bathe. A mist appears and covers them, and when it lifts they are no longer to be seen.
Millington, much to the puzzlement of Captain Bates, orders that all the radio transmitters on the base be disabled and that all chess sets be burnt.
Judson sets the Ultima machine to translate the runes. The phrase ‘let the chains of Fenric shatter’ is printed out.
Jean and Phyllis reappear, but they have been transformed into vampires. They entice a soldier into the water, where he is grabbed by a group of monstrous Haemovores (Ian Collins, Jennifer Crome, Ian Elliott, Perry Evans, Ann Graham, Raymond Martin, Jaqui Nolan, Tony Ryan, Graham Stagg, Cy Town, Alan Marshall, Tip Tipping, Paul Heasman, Sam Kent-Smith, Joe Kent-Smith). They then return to Miss Hardacre and kill her.
The two vampires next attack Wainwright at the church. They leave when the Doctor and Ace arrive, but threaten to return for Wainwright later. The Doctor discovers that Ace has given Judson the clue that he needed to solve the mystery of the runes: they are a logic diagram for a computer program. He, Ace and Wainwright hurry to the base to stop Judson from using this. They are too late, however, and Haemovores are summoned from the sea. Millington realises his mistake: the base has been weakened to allow the Russians to steal the Ultima machine and now they have no way of summoning help as he has also ordered that all the radios be destroyed – a task that Perkins (Christien Anholt) has carried out with an axe.
On the beach, Sorin and his men retreat as the Haemovores advance.
The two time travellers search the secret tunnels for what the Doctor describes as ‘something evil’. Ace finds an old flask and, thinking that it might come in useful, stuffs it in her rucksack.
The Haemovores try to break into the church, while the Doctor, Ace and Wainwright try to keep them out. The Doctor realises that faith will repel the attackers. He whispers the names of his past companions and this sets up a screaming noise that drives the creatures away. Sorin arrives and determines to rescue his men, relying on his faith in the Russian Revolution for protection. He gives Ace his scarf before leaving. The Doctor, Ace, Wainwright and two Russian soldiers escape through the tunnels, pursued by Jean, Phyllis and the Haemovores.
Ace wants to make some more nitro-9 using the old flask, but the Doctor realises that this is the ‘oriental treasure’ to which the runes referred.
The Doctor, Ace and Wainwright emerge from the tunnels to find Millington and his men waiting for them. Millington orders the exit doors barred, despite protests from the Doctor and Ace that there are still two soldiers inside.
Sorin tries to make a truce with Millington, but the Commander has him locked up.
Ace finds Kathleen sitting alone. The woman has received a note that her husband is missing, presumed dead.
Ace confronts the Doctor. She realises that he knows what is going on and demands that he explain. He tells her that Fenric, an evil intelligence from the dawn of time, is trapped inside the flask. They need help to defeat it, so Ace distracts a guard and thereby enables the Doctor to release Sorin.
Wainwright waits by the exit from the tunnels as the Haemovores start to burn their way out. The creatures emerge, and Wainwright is killed as his faith proves insufficient to hold them back.
The Doctor realises that Fenric needs a body to occupy. He and Ace hurry to the cipher room, where Judson is knocked out by a discharge of power from the Ultima machine. The Doctor, Ace and Sorin look on as Millington recites the Viking legend. Behind them, Judson rises to his feet, his eyes glowing green. He has been possessed by Fenric.
Fenric/Judson states that he has been trapped in the shadow dimensions for seventeen centuries, and that now his preparations are complete. He vanishes and reappears in the tunnels, where he orders Jean and Phyllis to fetch the Ancient One (Raymond Trickett). They go to Maidens’ Point and summon the creature from the sea.
The Doctor tells Ace that he needs a chess set to play his game with Fenric to its conclusion. They try to get one from Millington’s office, but he has mined it with explosives and they narrowly escape being killed. Ace then remembers that Kathleen also had a chess set.
As the Doctor hurries to set up the chess game, the Wrens are trapped by the Haemovores and transformed into vampires, following which they attack the soldiers. Judson’s nurse, Crane (Anne Reid), is also killed. Millington shoots down Sorin’s second in command, Vershinin (Marek Anton), but is then killed by Captain Bates. Ace sends Kathleen and the baby off in a car to stay with her Nan at 17 Old Terrace, Streatham. She then finds herself trapped by the pursuing Haemovores.
Fenric/Judson tells the Ancient One to take the poison into the sea and to destroy the other Haemovores. The Ancient One carries out the latter instruction, saving Ace from an attack by Jean and Phyllis, who crumble away to nothing.
The Doctor faces Fenric/Judson, challenging him to make the final move in the chess game that he has set up in the laboratory. He then convinces the Ancient One that to poison the seas would only bring about the destruction of the world – and that the future from which the Ancient One comes would no longer exist.
Ace finds Fenric/Judson hunched over the chess set. He cannot solve the problem of the final move. She runs off and finds Bates helping the wounded Vershinin. They are now working together. Realising that this is the solution to the chess puzzle, she returns to the laboratory to find Sorin. She tells him that the solution is for the opposing pawns to join forces. Sorin, however, has now been taken over by Fenric in place of Judson. The Doctor arrives to find that Fenric/Sorin has won. A lightning bolt hits the chess set and it bursts into flame. Fenric/Sorin reveals that all those involved are his pawns – including Ace, as Kathleen’s baby is her mother, whom she hates, and she has just created her own future. Fenric/Sorin orders the Ancient One to kill the Doctor and Ace. The creature is held back, however, by Ace’s total faith in the Doctor. Fenric/Sorin demands that the Doctor kneel before him, but the Doctor responds by telling him to kill Ace. He claims he knew all along that Ace was a pawn and accuses her of being an emotional cripple that he would never have allowed to travel with him if he hadn’t known the truth. Ace collapses. Now freed from the obstacle of her faith, the Ancient One herds Fenric/Sorin into the isolation tank and releases the poison, killing both of them. The Doctor grabs Ace and escapes before the room explodes. The Doctor tries to explain to a tearful Ace that he had to break her faith in him so that the Ancient One could act.
Later, on the rocks at Maidens’ Point, Ace is still troubled. The Doctor watches as she dives into the water. When she emerges, she says that she is not scared any more. They turn and walk away together.
Synopsis from Doctor Who: The Seventh Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted.