An Absolute, an agent for the information-gathering network called the System, arrives in 1930s Spain to record and document all events of the era. However, the nature of the System makes it a threat to Sabbath, for the System stores the facts of Sabbath’s escape from his underwater initiation. The System itself is impartial, but the knowledge it stores could be used against him, and thus the System will have to go. Sabbath supplies one of his own agents with glasses which will enable him to perceive the Absolute, and sends him hunting -- but as soon as the Absolute is observed, its nature changes, and Sabbath’s agent loses it.

The Absolute finds that it can no longer perceive the connections between the events it’s observing, and worse, it’s having trouble perceiving the person who first observed it. It is as though the man is both there and not there at the same time, which is absolutely impossible. Desperate, the Absolute begins to tap into human minds to collect data through them, but finds their perceptions contradictory and confusing. When it observes a conversation at a café, it experiences different, subjective versions of the same conversation. The young photographer Miquel Domínguez believes that the Englishman Eric Blair is entranced by his tales of derring-do; his sister Eleana is ignoring him, bored by the story’s macho embellishments; and Blair can’t speak Spanish well enough to follow the conversation, and is just nodding politely. The Absolute tries to pull Miquel into its own frame of reference to study him, but Miquel does not survive and the Absolute must return his body, shattered and pulped beyond recognition. Its link to the System then begins to deteriorate, and it finds that it is unable to upload the data it is collecting. It needs to find some way to store this data, or the contradictory and confusing perspectives will drive it mad...

The Doctor, Fitz and Anji attend the great Parisian exhibition of 1937, where they observe something odd about Picasso’s painting Guernica; although they’re unable to pinpoint any specific difference, the painting doesn’t carry any emotional impact. When Fitz compares it to a photo reproduction in a book from the TARDIS library, however, the photo holds all of the raw power which the original doesn’t. The Doctor realizes that the book is part of the TARDIS, a perfect copy manufactured when the ship reassembled itself -- which means that the book exists outside time and space, and is not affected by cultural context. Something fundamental about the way Western culture perceives history has changed, which could be a very dangerous thing; for example, what if everybody believed the version of history presented at the Nazi pavilions?

Determined to investigate, the Doctor drops off Fitz in Bilboa in April 1937, telling him to find his way to Guernica and observe the bombing. As this isn’t a welcoming era for a dark-skinned woman, the Doctor takes Anji to Barcelona, the least unsafe city in Spain, to see how news of the bombing is reported. However, he miscalculates slightly and ends up in November of 1936. There, he and Anji meet Eleana Domínguez, who is mourning her brother’s bizarre death; Eric Blair, an English novelist who has come to help his socialist comrades defend themselves against the fascist rebels; Pia Samscuro, an Italian Communist put to work as a translator by a Spanish commander who doesn’t believe women can fight as soldiers; and Jueves, a Hispanic correspondent for an American newspaper. Though early yet, the Doctor decides to use the TARDIS to scan for further perceptual anomalies before returning to April to meet Fitz.

The Absolute spots Anji and the Doctor, but sees them flickering in and out of existence, like the man who first saw it; they are both there and yet not there at the same time, and when it tries to tap into their minds it is rejected. As it taps into more and more people, however, it builds up a composite picture of Anji, one which will soon be close enough to the “true” version for it to strike. However, it finds itself unable to upload the data it is collecting into the System, and as the information piles up the overwhelming contradictions become too much for it to handle. It needs some way of “forgetting” or “ignoring” the inconvenient data, the way the humans do -- and while casting about itself, it finds a new link to an entirely different kind of intelligence, one with all of the memory space the Absolute needs.

Buenaventura Durruti, an officer in the Republican army, is shot and killed, but nobody seems to agree on how or why. Was he shot accidentally when his loyal aide’s gun misfired, or was he killed by a sniper? If the latter, was his killer one of his enemies, or one of his own men, a disillusioned anarchist who believed Durruti thought himself better than the men he was sending to their deaths? Eleana insists that Durruti was killed by fascists who believed he would lead the anarchists to victory, but Anji doesn’t know whose version to believe. As she chats with Pia, however, Anji suddenly begins to suffer from an increasingly painful headache, and when the Doctor arrives he tries to help her -- only to collapse in agony himself. Anji’s headache clears, but when she tries to get the Doctor back to the TARDIS, they find that it has shut itself down completely, with only a few dregs of emergency power available to them...

Over the next few months, the Doctor falls into what Anji can only describe as a sulk. Deeply disturbed by the possibility that he’s lost his TARDIS once again, he seems to be in a state of denial and insists that it will pull itself together. Anji tries to deal with the situation by researching people’s stories from the streets and looking for the bigger picture. People have reported seeing monsters in the streets, but the reports are contradictory and are shaped by the politics of the witnesses. For example, when an old architect is found dead in the ruins of the Sagrada Familia with a look of terror on his face, Eleana chooses not to report this in her anarchist newspaper, for fear that the public will draw foolish, superstitious conclusions from hearing of a mysterious death in an old church.

To snap the Doctor out of his funk, Anji suggests that he visit Eric Blair on the front lines. However, Blair is no longer there, having failed to return from his past leave. The Doctor questions Pia, who seems to know more than she’s saying; in fact, although she has come to like the Doctor and Anji, her superiors have ordered her to watch them and find out what their true agenda is. Wary of telling the Doctor too much, she tells him only that Blair isn’t the only person to go missing in Barcelona... and that some of those who do go missing don’t want to be found again. Frustrated, the Doctor returns to his hotel, trying not to be noticed by Times correspondent Kim Philby, whom he will meet for the first time in the future.

It is now April, and though the Doctor and Anji have spent months in Barcelona, Fitz has only been in Bilboa for a couple of hours. He tries to blend in and hitch a ride to Guernica, but a Russian named Sasha sees through his claim to be a sailor and kidnaps him. Fitz learns that the “humanitarian aid” Sasha is transporting to his comrades on the front is in fact weapons and ammunition, but as Sasha is going to Guernica, Fitz remains silent. Sasha explains that he’s curious to know who Fitz really is, and that he needs the company on the long drive. Fitz claims to be an independent observer, but Sasha claims that there’s no such thing. Fitz’s very presence is changing Sasha’s behaviour, as Sasha is trying to present himself as friendly so Fitz will write a favourable report on the Communists; and Fitz’s report will be coloured by his own perceptions in any case. Can any observer really be completely impartial?

Fitz takes an opportunity to sabotage one of Sasha’s tyres, which blows out just as they come within sight of Guernica. As Sasha tries to conduct repairs, he and Fitz witness the bombing. Guernica is destroyed in a riot of explosions and flame, and the fleeing civilians are gunned down mercilessly. Fitz and Sasha provide what help they can, but later, they compare stories and realize that they can’t quite figure out what happened to Guernica. It’s as though they saw three different versions of the bombing, all taking place at the same time. Did German Heinkels bomb the town and strafe the roads? If so, was this a deliberate attack, or were the planes blown off course by strong winds, their pilots confused by the smoke and dust kicked up in the initial bombing runs? Or, contradicting everything Fitz thought he knew about history, was the town actually bombed from within by retreating Republican forces, who destroyed their own town rather than let it fall to the fascists? Fitz is deeply disturbed, not only by this anomaly, but by the fact that Sasha seemed to see the same thing. The Doctor had told Fitz that only time-travellers, who aren’t tied to a particular time and space, can perceive such historical anomalies. Is Sasha more than he seems?

Back in Barcelona, Anji and Elena question a guard about his claim to have seen a monster, but he’s changed his story, presumably afraid of the other guards making fun of him. As Anji and Eleana return to the hotel, they are attacked by something which Anji can’t quite make out. Jueves arrives to rescue them at the last moment, and Anji faints, overwhelmed by the strange experience. Jueves and Eleana take her back to her hotel room, and while Jueves fetches the Doctor, Eleana looks about the room and finds Anji’s notes, including Eleana’s own claim that Durruti was shot by a fascist sniper. She is furious; anybody could read these notes and use them against her, and besides, they’re not even accurate. Everyone knows that Durruti was killed by his own men. Anji realizes that Eleana has changed her mind without knowing it, but as Eleana storms out of the room the telephone rings -- and there’s nobody on the other end. She and the Doctor are indeed being watched.

The Doctor and Jueves arrive with news that Blair was seen a few days ago in the company of Marc Rhein, an anti-Communist French journalist who has since vanished. However, the attack gives the Doctor a stronger need. Eleana, still furious, now denies ever having seen a monster and claims to have been attacked by the Guardia de Asaltos; however, Anji is unable to describe her attacker in any detail. As Anji is sensitive to historical anomalies, this confirms that there is something extraordinary in Barcelona -- and as the TARDIS shut down after the Doctor programmed it to search for anomalies, perhaps it did so to protect itself from something it found, rather than because of an internal fault.

Elsewhere in Barcelona, the Absolute is in terrible shape. When its earlier plan failed, it tried to possess one single human and strip away all of the emotional and perceptual filters which caused the man to selectively edit reality. This plan failed as well, and the Absolute, overwhelmed, has tried to create its own filters. It has given itself form, a shadowy copy of the man it tried to control, and has named itself Enrique. But it is still unable to handle the overwhelming contradictions in perception, and is trying to whittle the different versions of reality down to one “true” version by stripping away all of the conflicting data.

The Doctor and Anji collect the conflicting reports on the Guernica bombing, and slowly boil down all of the stories to one consensus version -- the retreating Republicans deliberately bombed their own town. Even loyal anarchists and socialists are beginning to believe that this is true. As they wait for Fitz to return, Jueves finally confronts them, demanding to know what their real agenda is. He storms out when they are unable to admit the truth. Anji wants to do so, but the Doctor is distracted when he leafs through The Age of Reason -- the book with the photo of Guernica -- and sees scribbled notes in the margins. When Anji confirms that this is Fitz’s handwriting, the Doctor seems horrified...

Fitz and Sasha catch the train back to Barcelona, and Sasha goes to check in at the Communist Party headquarters while Fitz tracks down the Doctor and Anji. When he tells them what he saw, they realize they’ve been stuck in Barcelona for so long that they too have become affected by the culture and nearly succumbed to the consensus version. The Doctor is disturbed when Fitz admits that Sasha also saw conflicting versions of reality, and when he tries to question Sasha he finds that Sasha never showed up at the Communist Party headquarters. He asks Pia for help, but Pia, who has been transcribing “interviews” conducted by the new interrogator, Burton, is too frightened to help. Fitz reluctantly concludes that his new friend had been lying to him, and unfortunately he’s right; Sasha is Sabbath’s agent, the one who first tried to track down the Absolute. Observation of the Absolute changed its nature, and those changes have rippled back into the System, causing massive damage -- and thus destroying the records Sabbath wanted kept secret. However, the Absolute still poses a threat, and thus, when Sasha met Fitz and recognized him as the Doctor’s companion, he used Fitz to lead him to the Doctor. Now, Sasha contacts Sabbath and asks him to send him a few months back in Time, so he can work on getting close to the Doctor and using him to deal with the Absolute.

Anji follows up a lead on Blair’s disappearance to the telephone exchange, and although the lead itself goes nowhere, she does find something more relevant behind a locked door in the attic -- two versions of the same room, one of which contains images like photographs floating in mid-air. The pictures are of herself, Fitz, and the Doctor -- and to her surprise, some of the pictures show Fitz and Jueves together, although Anji didn’t think they’d met yet. Anji is so upset by the thought of being watched without her knowledge that she doesn’t notice fighting has broken out, months of tension between the different factions finally erupting in violence. She nearly walks right into the middle of a gunfight before she realizes what’s going on, and although the Guardia de Asaltos rescue her from the immediate danger they then arrest her and throw her into a cell with the other fighters. She does not resist, believing that they will release her once they examine her papers and realize that she’s a civilian.

The phones in the hotel go down due to the fighting at the exchange -- but the Doctor’s phone rings anyway, and when he answers there’s nobody on the other end. He finally realizes that since he programmed the TARDIS to scan the local media sources, it must have connected itself to the phone system -- and deduces that it shut itself down to protect itself from something it found there. He returns to the TARDIS, jury-rigs a tickertape printer and connects it to the console, and finally gets a hard-formatted copy of the data that’s been waiting for him for months. He goes to the exchange to investigate, but on his way he runs into Eleana -- and when he tells her what he’s doing, she tries to shoot him, apparently having decided that he’s a fascist. Her gun misfires and blows up in her face, and the shock clears her head, causing her to realize that something has been in her mind for months, altering the way she sees things.

The Doctor gets Eleana to a medic and goes to the exchange, where he tracks down the strange attic room and finally meets Enrique, whose pieced-together human form seems vaguely familiar. The Absolute is now quite mad, and it panics and flees when it sees the Doctor. Although the others in the exchange don’t see Enrique, they do see the Doctor and perceive him as a monster. The Doctor deduces from the Absolute’s mad babble that it has been collecting information by tapping into other people’s minds -- but that its subjects’ perceptions have been changed by the Absolute’s observation of them.

Fitz visits the offices of the POUM to look for Sasha, hoping that his friend has just gone into hiding from another hostile faction of the Party. When the fighting breaks out, he is forced to help man the barricades for fear that the others in the building will turn on him if he refuses. The fighting finally dies down, and Fitz goes out for drinks with the others from the POUM -- but when officers from the People’s Army enter the bar, the mood grows ugly and a fight soon breaks out. Fitz extricates himself from the brawl only to be arrested and thrown into a cell to await a meeting with Burton. People who go before Burton are never quite the same afterwards, if they’re ever heard from again. When Fitz is dragged out of his cell, he sees the Doctor’s friend Pia waiting outside -- but she pretends not to know him, and does nothing as he is taken away for interrogation.

The Doctor returns to his hotel, only to find that Anji’s notes have been stolen and that there is no sign of her or Fitz. Outside, he meets Jueves, and admits the truth about himself and his friends. He has deduced that the Absolute, overwhelmed by the data it had collected, tried to use the TARDIS’ connection to the phone system to use it as a storage system; the TARDIS shut down to keep it out, and, desperate, it simply ejected the data which contradicted its “accepted” version of reality. The people it collected these viewpoints from have forgotten that they ever held them, and the mass of contradictory data has taken form as the “monster” which people have seen roaming the streets. Only time-travellers such as Anji can see the creature as a mass of contradictions; everyone else simply imposes their own perceptions upon it and sees it as the one thing they most fear. The Doctor then learns that Fitz has been arrested, and when Jueves learns that Fitz was placed in this position because he was looking for his missing friend Sasha, he offers to try to help rescue him. As a foreign journalist, an independent he stands a chance of success, as Fitz’s captors may choose to show him a friendly side of themselves which they want the outside world to see. The Doctor gives Jueves a set of instructions for Fitz, and sets off to locate the “monster”, which he believes has been looking for him all along.

Mentally and physically exhausted by his “interview” with Burton, Fitz is barely able to react when Sasha shows up and orders the other Communists to release Fitz into his custody. Once clear of the Party headquarters, Sasha assures Fitz that this is a rescue, and passes on a set of instructions from the Doctor. Fitz must use the TARDIS’ external phone, which is now linked to the Absolute’s System, to contact von Richthofen and convince him to order an air attack on Guernica. This will create a paper trail, physical proof of the Germans’ involvement, ensuring that people can find out what really happened rather than accepting the consensus view that the Republicans are responsible. Fitz is unsure whether he can order an attack on civilians just to prove that the attack actually took place, but he goes to the TARDIS anyway, knowing that the Doctor trusts him. However, Sasha realizes that Fitz is too weak to risk connecting to the System, and, blaming himself, Sasha uses the TARDIS phone himself, leaving Fitz unconscious in front of the TARDIS as the ship draws him into the System.

The Doctor, realizing that it’s the offcast-creature who has been trying to contact him via the phone system, locates it by picking up a public phone and asking it to show itself. It emerges from the phone receiver as a mass of discordant shapes and angles, aspects of the viewpoints which Enrique had rejected. The Doctor recognizes part of the creature as Eric Blair -- or rather, the parts of Eric which Enrique had rejected. The creature explains that Enrique tried to possess Blair completely by stripping away all of his perceptual filters, but this created an amoral personality which rejected Enrique. Enrique has nevertheless used Eric’s body as a template for its human form, which is why the Doctor found it so familiar earlier. Eleana was being used by the Absolute, but she is now free of its control and has fled from the city.

The Doctor and the creature set off in search of Enrique, and find him in the Sagrada Familia. Enrique is terrified of the uncertainty which the Doctor represents, and can’t even bear to look at the creature, which represents all of the viewpoints he has rejected. The Doctor warns Enrique that he, the Doctor, is already on Earth in several places at the same time, and will be for several more decades -- but the Doctor can offer Enrique a way out. He tears the cover from Fitz’s book, and uses a series of cuts to unfold it into a loop of paper large enough to surround himself, Enrique, and the offcast-creature. Since the book is part of the TARDIS, the loop transports them back to Guernica, where Enrique sees the German bombers for himself. By denying this version of reality, Enrique has denied the reality of the deaths and robbed them of meaning. The creature then envelops the disoriented Enrique, the two merge into one, and the sheer mass drops the Absolute back out of history. As it departs, the viewpoints and perspectives which it had rejected return to those from whom it had collected them...

Anji is taken for interrogation, and when Eric Blair enters the room she assumes that she’s been rescued -- until she notices that his nametag reads “Burton”. Blair slaps her in the face, calls her a “stupid black bitch,” and demands to know who she’s working for -- and then collapses in shock as the Absolute departs, finally allowing him to see for himself what he’s been doing. Though shaken by his abuse, Anji decides to trust him when he promises to help her escape. However, they pass Pia on the way out, and Pia, seeing Anji’s condition, decides that she’s been fighting for the wrong side and holds “Burton” at gunpoint, demanding that Anji be released. The building’s guards open fire, and Blair is shot in the throat as they escape. Pia decides to return to Italy, while Anji returns the injured Blair to his colleagues and arranges for him to be returned to England with his wife.