France, 1819: while returning to London from Paris, English scientists Charles Babbage and John Herschel meet William Kempton, a man who takes exception to their belief that it may be possible to create a steam-powered calculating machine -- and that night, Kempton murders them both. America, 1884: patent clerk Herman Hollerith returns home from work to find that his prototype calculating machine has been smashed to pieces, his wife has been savagely beaten to death, and he stands accused of the crime. Germany, 1941: Konrad Zuse’s friend Helmut Schreyer tells the Nazi high command of Zuse’s groundbreaking work on a cipher machine, but the message is intercepted and British bombers make a suicide run on Berlin to destroy Zuse’s work...
17 April, 2003. The fragmentation of history caused when Sabbath attempted to re-engineer the timelines in Siberia has apparently been resolved by the infinity process on Selonart. The Doctor therefore returns Anji to 21st-century Earth, materialising in Edinburgh. Anji bids her friends farewell and sets off to look up her co-worker Mitch, while the Doctor and Fitz split up to explore, arranging to meet at the local tea rooms at noon. Fitz buys fish and chips from a vendor who, strangely, doesn’t accept decimal currency, and is amused by odd headlines in the paper which he assumes are there as a joke... but this is not in fact the world he knows. In London, a childlike figure called the Oracle senses the arrival of the TARDIS, and sends its adjutant, Rameau, to inform the Star Chamber, the cabal of five men who rules Britannia behind the scenes. Its leader, the Pentarch, orders that the blue box be brought to London.
Trying to find Mitch’s office building, Anji instead finds a junkyard at the address he’d given her. When she asks passers-by for help, she encounters such shockingly casual racism that at first the other anachronisms and bizarre comments fail to sink in. Unable to get directions to the airport, she finds her way to a crowded railway station with long queues and no automatic ticket machines. The ticket clerk has never even heard of a “credit card,” and casually tells Anji that she’ll have to ride with the luggage so as not to offend decent white folk. Shaken and furious, and slowly coming to realise that this is not her world after all, Anji leaves the ticket office to track down the Doctor and Fitz.
Fitz sees a striking red-haired woman leaving an auction house, and later runs into her at the tea rooms, but on both occasions she deliberately avoids speaking to him. Anji also bumps into the red-haired woman, who seems to recognise her and flees without speaking to her. Anji joins Fitz in the tea rooms, but the manager, Francis Clooney, orders her to leave, as her presence is upsetting his white customers. As Anji argues with him, Fitz notices a man leaving the tea rooms without his Gladstone bag, and follows him out to let him know -- but moments later the bag explodes, blowing the tea rooms to bits and burying Anji in the rubble. Fitz pursues the bomber, but the man fights him off and escapes. When Fitz returns to the rubble to look for Anji, he is accused of planting the bomb himself, and nearly beaten to death by two policemen before special forces arrive and arrest him.
Realising that something is wrong, the Doctor has visited the city’s reference library to look for historical anomalies, but he is suffering from dizzy spells. His odd behaviour attracts the attention of young librarian Hannah Baxter, who determines that he’s interested in science and invites him to attend a secret meeting with her friends after closing hours. The Doctor returns to the tea rooms, but on the way he sees a party of labourers loading the TARDIS onto a lorry. Acting under orders from the Security Service, the foremen ignore the Doctor’s demands for an explanation and drive off with the TARDIS. The Doctor is too late to prevent them from loading the TARDIS onto a train bound for London. He then hears the explosion of the tea rooms and rushes to investigate. On the way he bumps into a man named Edward Knox, who has just resigned in disgrace after a striking red-haired woman stole a valuable ring from the auction house where he worked.
Paramedics unearth the gravely injured Anji from the rubble, and although most are willing to set her aside while they continue looking for white survivors, Dr. MacLeod insists that she be taken to hospital. The Doctor then arrives, but is forced to take his leave before the police connected him to Fitz. Fitz himself is beaten repeatedly during his interrogation, and when a lawyer named William Hastings arrives, Fitz tells Hastings all he knows, giving Hastings full descriptions of his friends so they can confirm his story. However, Hastings promptly hands over the descriptions to the security forces, identifying the Doctor and Anji as Fitz’s fellow terrorists, and leaves the security guards to beat more information out of him.
Following word of the bombing in Edinburgh, the Pentarch summons Prime Minister Daniel Merrell before the Star Chamber and orders him to impose martial law and a twelve-hour curfew. Though Merrell is aware that public sentiment is against him, he also knows that he cannot disobey the Star Chamber, and reluctantly agrees to call Parliament back into session on Good Friday to push through the bill. Elsewhere, the Oracle waits for the elemental to arrive in search of his missing blue box...
Unsure what to do about Fitz and Anji, the Doctor keeps his meeting with Hannah, who takes him to a relatively liberal pub named the Living Room to meet like-minded friends. In this world, creative education is discouraged in favour of rote learning, but here, a number of disgraced educators continue to meet and discuss new and critical ways of thought. Professor Hamilton initially welcomes the Doctor to their circle, but rejects Hannah’s suggestion that they should stop talking and start acting to overthrow their oppressive rulers. He reminds her of the tale of the “shroud,” a legendary blueprint of a machine which could change the world, and insists that thought rather than violence is the way forward -- but Hannah suspects that he and his fellow educators in fact merely fear reprisals.
Anji is suffering from a severe concussion, and MacLeod has her kept under observation in the ward. As she ponders the lack of life support machines, a Security agent arrives to arrest her, but is turned away by the ward sister. Elsewhere, however, Hastings informs Fitz that they have Anji captive and will let her die from her injuries unless he confesses to his crimes on live television. Fitz reluctantly agrees to do so when he realises he will be executed on the spot if he refuses.
The broadcast is seen both in the hospital and in the Living Room. Fitz, clearly prompted from off-camera, confesses to the bombing, identifies the Doctor and Anji as his associates and begs them to turn themselves in. In the ward, the injured Francis Clooney recognises Anji, and the sister is forced to move her to a private room before the other patients can lynch her -- but she too believes Fitz’s confession and vows to turn Anji over to Security when they return. Anji is too badly concussed to escape, and loses consciousness when she even tries to stand up. Fitz is returned to Edinburgh Castle for more beatings, while Hastings receives a commendation for his work and a promotion to a special assignment in London. All Fitz can do is hope that the Doctor saw the broadcast and is working on a plan to rescue him and Anji.
Hamilton and the others, appalled by what they’ve heard and terrified of reprisals, expel both the Doctor and Hannah from the circle. Hannah, infuriated by their treatment of her, accepts the Doctor’s word that his friend had a false confession beaten out of him, and shows him to Edinburgh Castle; however, the Doctor realises that it’s far too well-guarded for him to risk rescuing Fitz. Instead, he decides to search the hospitals for Anji. Hannah accompanies him, but notes that he is still suffering from dizzy spells and disorientation -- which seem to grow worse whenever he thinks about the distortions in the timeline. Meanwhile, Hastings leads a raid on the Living Room, and despite Hamilton’s protests and attempts to buy his friends’ lives with information about the Doctor, the security forces shoot them all, including the landlord who allowed them to meet.
18 April: Good Friday. The Doctor and Hannah finally track down Anji in Leith Hospital, where the exhausted Dr. MacLeod accepts the Doctor’s claim to be a specialist from the Royal Infirmary and orders the suspicious ward sister to give him the key to Anji’s room. The Doctor and Hannah then smuggle Anji out of the hospital moments before the Security agent returns for her. Furious, the agent arrests MacLeod for collaborating with terrorists; he will be tortured and interrogated in Anji’s place.
Fitz’s trial is held at the High Court Justiciary, where his attempt to retract his confession is stricken from the record. Francis Clooney tells his story to the court, and an undercover Service agent then arrives to deliver eyewitness testimony. Placed behind a screen so that his identity will not be compromised, “Mr. R.” tells the court that he saw Fitz plant the bomb in the tea rooms, and that when he left to raise the alarm, Fitz chased him out and tried to kill him. Fitz catches a glimpse of Mr. R. and recognises him at once as the real bomber. However, his claim that the Security Service is faking terrorist activities to keep the people in line is scoffed at, and he is sentenced to death for treason.
The Doctor and Anji hide out in Hannah’s flat, where they hear the news of Fitz’s trial; having been found guilty, he is to be sent to London for further interrogation. As Hannah leaves to arrange transportation to London, the Doctor tells Anji that the TARDIS has also been taken to London -- perhaps as bait to lure the Doctor there. He and Anji compare notes on what they’ve seen, and conclude that the computer was never invented in this world. Without computers to aid in their design and operation, there are no jet planes or spacecraft, satellites or mobile phone networks, and no Internet. Thus, there has been no globalisation, and the world is still made up of relatively isolated communities who regard each other with incomprehension and fear. And the Doctor fears that society has been engineered this way...
Hannah learns of the slaughter at the Living Room, and insists upon accompanying the Doctor and Anji to London, as there’s no reason for her to stay in Edinburgh. One of her friends smuggles them out of the city in a van carrying carrots to Manchester, and on the way, the Doctor asks Hannah more about the “shroud” which Hamilton had mentioned. He soon realises that the shroud she describes is a blueprint for a calculating device -- one of the Universal Machines which the Doctor’s old friend Alan Turing once hypothesised. In this world, Turing was charged with sexual deviancy in 1936, and has apparently been imprisoned in the Tower of London ever since. His legendary design, a holy grail amongst the scientifically inclined, is now referred to as “the Turing Shroud.” The Doctor realises that whoever has suppressed the invention of the computer must have had foreknowledge of the historical events which needed to be changed, but again, he suffers a fit whenever he tries to think about it.
In London, Merrell pushes through a bill imposing drastic new security measures on the country. Meanwhile, the Oracle studies the TARDIS and orders Rameau to find a way to break in before the elemental returns for his property. Rameau reports to the Pentarch, who concludes that the “Doctor” identified as Fitz’s collaborator will now be on his way back to London to retrieve his friend and his property. But the Pentarch is sick of relying on the predictions of the freakish Oracle to maintain his beloved Empire...
19 April. The Doctor, Anji and Hannah hitch a ride from Manchester to London with Alf, a trade unionist planning to join a mass protest against the government on Easter Sunday. On the way, the Doctor admits to Anji that Alan Turing was a great friend, and more, during the Doctor’s time alone on Earth -- and that if he puts history to rights, this will mean restoring the version in which Alan died unhappily. Alf drops his passengers off near King’s Cross, and Hannah leads the Doctor and Anji to an empty building in Tavistock Place to make contact with a resistance cell Hamilton had heard about. There, they meet a man named Frank and a belligerent young woman named Dee, who question the newcomers and eventually agree to join forces with them in their fight against the Star Chamber.
Fitz is transported to the Tower of London, where he finds that his nemesis Hastings has been appointed the new prison governor. Fitz is locked up in a cell next to an elderly man named Alan, who claims to have been imprisoned alone for decades. Though wary of another trick, Fitz nevertheless sympathises with Alan and shares his story with him. In turn, Alan claims to have been arrested and placed in solitary confinement decades ago; he’s been kept alive ever since, and his jailers even force-fed him when he went on a hunger strike. Hastings then arrives and informs Fitz that his execution has been scheduled for Monday.
Frank tells the Doctor about the legendary Star Chamber which rules the country, but remains optimistic that such a repressive regime can’t last forever. He leaves to attend a meeting with other resistance members, telling the others to stay put, but Anji gets fed up with doing nothing and convinces Dee to take her and her friends out for a pint. At the pub, Dee reveals that the resistance has learned that the shroud is supposedly kept in the Tower of London, but the Doctor suspects that this information has been leaked to them. His face then appears on the news, and although he and his friends get out of the pub before anyone else sees them, the landlord recognises them and telephones the information hotline. Back at the flat, the Doctor collapses, still suffering from convulsions, as if his body is automatically responding to a threat he can’t consciously perceive. The others try to get some sleep before their big day, unaware that the Doctor’s fears are legitimate; the Pentarch knows that the Doctor intends to rescue Fitz, and has leaked the location of the shroud to lure the resistance into a trap. The resistance will be crushed completely within 24 hours.
20 April: Easter Sunday. The Doctor recovers from his fit, and advises Hannah not to blame herself for the deaths in Edinburgh, admitting that he once did something so terrible that he can no longer even remember what it was. Dee then receives word from her friend Bill, who has overheard the Tower’s new governor, Hastings, planning to move the shroud to another hiding place. Hannah urges Frank and Dee to attack the Tower today while they still know where the shroud is, acquiring the shroud and rescuing the other prisoners, including Fitz. Security forces then arrive at the flat, alerted by the pub landlord, but Dee has rigged the building to explode, covering her and her friends’ escape.
The trade unionists, including Alf, march through the streets of London to Trafalgar Square, unaware that Merrell has ordered their deaths. The Doctor, Anji and Hannah see policemen and soldiers marching to the square and realise that the protest is going to become a bloodbath, but they can do nothing to stop it. They are reunited with Dee and Frank at the Alcove Cafe, but while there they are locked inside by policemen who explain that all ordinary citizens are being confined inside for their own safety until the unionists’ demonstration is over. The Doctor and his allies are stuck inside the cafe for hours, forced to watch helplessly as the unarmed, peaceful protestors -- including Alf -- are gunned down without mercy.
Alan tells Fitz about the death of his first love, Chris, from tuberculosis. Alan continued with his life’s work, but fell afoul of the Security Service when he published a paper on the possibility of constructing mechanical calculating machines. His friend Robert, forced to frame Alan for seducing young men and enlisting them into subversive organisations, killed himself during the trial. Hastings then has Fitz brought before him to question him about what his friends are planning, and when Fitz learns that the Doctor has rescued Anji and is on his way to London, he bursts out laughing, telling Hastings that he’s brought the Doctor’s wrath down upon himself. Hastings, furious, sends Fitz back to his cell -- and for the first time in decades, on orders from a higher authority, Alan is removed from his cell and taken elsewhere.
The Oracle gives Rameau an alien metal tube which he can use to hurt the TARDIS, and Rameau has it delivered to the laboratory where the TARDIS is being examined. Professor Joshua Sutton is startled when a beam of black light emerges from the tube and cuts through his workbench, but nevertheless turns the mysterious cutting beam on the TARDIS as ordered. Meanwhile, the Star Chamber observes broadcasts of the massacres which have taken place in Trafalgar Square and other cities across England, and are satisfied that order has been restored.
The dim-witted but well-meaning Constable Neil Judd releases the Doctor and his allies from the cafe, failing to recognise the Doctor from the news. The Doctor and his friends set off to get into position for the attack on the Tower, but the Doctor then collapses in agony, hearing the TARDIS scream in his mind as it is attacked -- and hearing a vaguely familiar woman’s scream mixed in with the sound. When the Doctor recovers, he tells Anji that he has to do something more important than rescue Fitz, and Anji, furious, accuses him of betraying and forgetting his friends. The Doctor in turn reminds her that she’s betrayed her friends before, and, even more enraged, Anji storms off to help the resistance rescue Fitz while the Doctor sets off alone, following the sound of the TARDIS’ screams.
Frank, Dee, Hannah and Anji arrive outside the Tower, and as they wait for the other resistance cells to create a diversion, they see the guards escorting someone or something away from the Tower. An explosion at Fenchurch Street station then draws away more guards, and their inside man, Bill, lets Dee and her friends into the Tower. Hastings, finding himself understaffed and under attack, goes to Fitz’s cell to demand an explanation, but Hannah and Anji find him and overpower him. Dee then forces Hastings to take them to the room where the shroud is kept, and once there she kills him only to find that the room is empty, mere bait for a trap. Frank then reveals himself to be a double agent and tries to arrest Anji and Fitz, but Dee shoots him -- and Hannah then shoots her, revealing that she too is an undercover Service agent. She only had a minor posting in Edinburgh, but when she met the Doctor she recognised his importance and stayed close to him in order to deliver the resistance leaders into the Service’s hands and prove her worth to them. Before leaving, she plants a bomb in the “shroud” room; as they did in Edinburgh, the Service intends to stage a terrorist attack in London to keep the populace fearful of non-existence external threats, and thus cowed, in line and under control.
The Doctor turns himself in at Whitehall, where he is taken to explain his actions to the Star Chamber. They claim that they are protecting humanity from the horrors of change, preventing the development of weapons which could destroy the world and preserving a decent way of life. The Doctor eventually realises that they genuinely believe what they’re saying, and such a twisted view of reality can only mean they’re being manipulated by someone else. Rameau then emerges from the shadows, identifying himself as Sabbath -- but this Sabbath appears older than the man known to the Doctor, and he only recognises the Doctor by reputation. This is not the Sabbath whom the Doctor has met before, but the alternate version from this timeline.
The alternate Sabbath unleashes his monstrous apes upon the members of the Star Chamber, who have served their purpose. He then takes the Doctor to the Jonah, which is docked in the shadows of the Houses of Parliament, and there, he introduces the Doctor to the Oracle, a thing from outside time and space which has helped him to engineer this version of history. Hannah arrives with Anji and Fitz, but on the Oracle’s command, Sabbath shoots her. He then has Alan Turing brought in, and as the Doctor begins to convulse he realises that it’s not the Oracle’s presence which is casing his spasms -- it’s Alan himself, who has become the focal point for the distorted history created by this alternate Sabbath.
Sabbath explains that the Time Vortex has become unstable due to the loss of the elementals who once kept it under control. Soon it will shatter completely, the separate timelines will merge together, and forces from outside the Vortex will swarm in to feed upon reality. With the help of the Oracle, Sabbath has generated a historical nexus which hinges upon Alan Turing; when Turing dies, the focal point will collapse, separating the Earth’s history from the rest of the Universe and protecting it from the horrors beyond the Vortex -- evils which the Doctor has seen before in the form of a great black eye staring down upon creation.
Or so Sabbath believes. In fact, the Doctor realises that the Oracle is one of these horrors it has warned Sabbath against, and that it has come to this reality to feed. In this version of history, the Oracle rescued Sabbath from drowning during his initiation; in the Doctor’s timeline, he was rescued by the creatures which the Oracle has described as monsters. In other words, whatever the “real” Sabbath is planning, this alternate version has been tricked into undoing. The Doctor tries to warn Sabbath that he has been deceived, but Sabbath ignores him and shoots Turing -- and reality shatters about Turing as he dies, collapsing into a darkness like a dark, staring eye.
The Oracle attacks the Doctor, having lured him here to feed on the life force of an elemental and his machine and survive the destruction of this reality, but the horrified alt-Sabbath intervenes. The Oracle transforms into its true, horrific shape and tears the alt-Sabbath apart, but while it is distracted, Anji and Fitz get the Doctor into the TARDIS and escape, leaving the Oracle trapped in a reality which is now falling apart around it; however, the Doctor fears that the collapse of this timeline will spread to the timelines which surround it and then on to all others. He tries to prevent this by travelling back to 1762 and rescuing Sabbath before the Oracle can get to him, but fails to reach him in time, and history collapses before he can try again. Fortunately, the Doctor discovers that the paradoxical presence of Anji and Fitz, who originated in another version of reality, softened the blow when the focal point collapsed. The cataclysmic domino effect which he feared has not yet begun, but the damage has been done, and the entire Universe is one step closer to total destruction unless the Doctor can restore the real version of history in time...