Doctor Joyce Brunner has just placed her mother Norma in an experimental Alzheimer’s treatment programme at Graystairs, a clinic near the Scottish village of Muirbridge. Dr Menzies claims that Norma is responding well, but Joyce suspects that all is not as it seems here; ashamed of her own feelings of frustration, she rushed her mother into this programme without doing enough research on its background, and now she’s starting to notice disturbing little details. The caregiver Megan seems to lose her temper too easily to be suited for this type of work, and she reacts particularly strongly when one of the patients, Eddie, apparently wanders off into the woods. After another strange incident, Joyce sends a letter to her old friend from UNIT, the Doctor, asking him to drop by -- but before he arrives, she stops by Graystairs again to visit her mother, only to find a strange, balding albino man in the room with her. Before Joyce can react, someone strikes her in the head, knocking her out...

The Doctor and Ace visit London in the year 2012 to pick up the Doctor’s mail from the Countess Gallowglass, a woman who runs a mail-forwarding service for aliens and time travellers. Before the Doctor leaves, she tells him that her cat is doing well, and that he should avoid the area on the following July 4th. The Doctor then finds something disturbing in his mail, and returns to the TARDIS with Ace, telling her to fetch a book on Alzheimer’s from the TARDIS library. As she does so, the TARDIS lands and takes off again almost immediately, and when she returns to the console room she finds that the Doctor has jammed the door shut and appears to be speaking to someone on the other side. Concerned, Ace tries to break in, but the Doctor drops off his mysterious passenger before she can do so, and refuses to tell her what’s just happened. For her, it hasn’t happened yet. The TARDIS then materialises in Muirbridge in April of 1982, and the Doctor and Ace head into town to investigate, unaware that they’re being watched...

The Doctor explains about Joyce’s letter, but he can’t tell Ace whether the clinic has really found a cure for Alzheimer’s; if she knew about her future, she might be tempted to change it. Unable to find Joyce, they split up; the Doctor checks Joyce’s room at the B&B, while Ace asks around town. She gets nowhere, but does meet a hiker named Michael who departs abruptly when she mentions that she and the Doctor are in town to look for Joyce Brunner. Having learned only that several sheep have recently gone missing, Ace decides to check out Graystairs, where she finds Norma Brunner sleeping soundly and meets a rattled young caregiver named Claudette. According to Claudette, many staff members have recently departed; Claudette herself plans to go, as she has been somewhat shaken by her recent encounter with a bald albino man who may be the clinic’s new owner, Mr Sooal.

Ace returns to town and waits for the Doctor at the pub, where she meets the bartender Claire and a strange middle-aged couple with a small dog. When the Doctor returns, Claire assumes from his strange behaviour that Ace intends to check him into Graystairs. The Doctor reports that he’s found no sign of Joyce -- but he did meet an elderly man whose eyes appeared to be subtly inhuman. He and Ace return to Graystairs and break into the cellar, where they find a chemical vial with lip marks on the side, evidence that someone has been drinking something toxic to humans. Now convinced that Joyce was right to be worried, the Doctor and Ace return to town, but on the way back Ace gets the impression that they’re being followed. The Doctor dismisses this as jer imagination, and dismisses her, claiming that he wants to investigate alone -- but as soon as she’s gone, he returns to the TARDIS to have a few sharp words with her “stalker”. Having made his point, he sets off to investigate the reports of missing pets and sheep, and eventually locates an isolated cottage containing the remains of the animals and a fleshsuit tank -- an invention of the Fleshsmiths which has been used to reconstruct the flesh of the dead animals into a disguise, to enable someone or something to pass as human.

Back at Graystairs, the elderly resident Harry finally begins to remember the details of his past life -- and the memories of death and carnage are glorious. His friend George is worried by the change in Harry, and he’s right to be; that night, to celebrate his recovery, Harry murders his fellow resident Doris Wesley, smothering her with a pillow and stabbing her in the ear with her own knitting needle.

While talking to Claudette in Norma Brunner’s room, Ace saw a photo of the hiker Michael on the bedside table -- and Michael was dressed in UNIT fatigues. Realising that Michael is Joyce’s son, Ace tracks him down to his campsite, but again, he clams up when she mentions the Doctor, and refuses to explain why he’s here. She eventually gives up and returns to Graystairs to investigate further, and while investigating the cellar kitchen, she finds a half-eaten bacon sandwich and hears a commotion in the stairwell. Trying to escape, she inadvertently passes through a transmat beam and finds herself in a badly damaged spaceship, where the missing staff and residents from Graystairs have been strapped into couches and fitted with metal implants. Ace recognises one of the victims as Joyce from Norma’s photos, and disconnects her, abruptly waking her from a dream of mathematics. Disoriented and confused, Joyce nevertheless helps Ace to free the elderly sisters Connie and Jessie from the machine, and helps them back through the transmat to Graystairs while Ace remains to free the others from the machine.

The Doctor returns to Graystairs, where he finds that Doris has been killed, and meets Norma. She can’t recall exactly what’s happened, but she does know that Joyce is in trouble, and that the bald, albino Sooal has been talking about a woman named “Stacy Chambers”. Meanwhile, Sooal discovers that someone has been disconnecting people from the processor on the ship, and when he heads for the transmat he finds Megan recovering in the stairwell, where someone hit her with a frying pan. Convinced that her attacker was Ace, Megan sets off to track her down while Sooal investigates the intrusion on the spaceship. There, he finds Ace and tries to kill her, but Ace flees to an airlock; there, she discovers that the spaceship is hidden underwater, and thus dons a spacesuit and escapes before Sooal realises what she’s doing. Back at Graystairs, Ace’s “stalker” hides and waits for the proper moment to act...

Joyce leaves the shaken Connie and Jessie in their room and returns to her B&B, where she is reunited with the Doctor -- although it takes her some time to get used to the fact that this new incarnation is the same man she knew from UNIT. She explains the circumstances which led her to ask for his help; in the woods outside Graystairs the other day, she encountered an old man who claimed that they had been putting evil thoughts in his head. When she told Megan what had happened, Megan became strangely furious, and Joyce later overheard her discussing the situation with a man who claimed that all they had worked for would be ruined without Eddie. The Doctor realises that “Eddie” must be the same old man he met in the teashop the other day -- the one who wasn’t entirely human.

Planning to investigate further, the Doctor removes the implant from Joyce’s neck and returns her postcard, telling her to give the correct date and time the next time she writes; he had to go to a lot of trouble to track her down. While Joyce rests and recovers, the Doctor returns to Graystairs and takes the transmat to the spaceship, where he uses Joyce’s implant to connect himself to the processor in order to find out what it does. Unfortunately, his assumption that the victims had been drugged is incorrect -- the implants actually disable the victims’ nervous systems, and the Doctor finds that he is unable to disconnect himself as the implant tries and fails to adjust itself to his inhuman brain signals...

Ace surfaces near a boat owned by two brothers, John and Alexander, who reveal that she is now in the Orkney Islands. John is conducting research for his PhD, but all of his electrical equipment has failed due to interference from beneath the water. While investigating, he found a huge silver dome on the seabed, and removed a small device from its surface in order to examine it more closely. However, he doesn’t believe Ace’s claim to have travelled by transmat to an alien spacecraft, especially when she realises that his description of the dome doesn’t match the ship she just escaped from. There are two things down there. And with no way back down, she’s stuck here, three hundred miles away from Graystairs and the Doctor.

Sooal, enraged by Ace’s escape, returns to Graystairs and orders Megan to kill her; he’s too weak to do so himself, and if the processor doesn’t do its work soon his condition will kill him. Megan takes the transmat to the ship, where she sees the Doctor and discovers that the implant is killing him. She leaves him where he is, knowing that his brain will burn out in less than half an hour. But it’s finally time for Ace’s “stalker” to act, and the figure shows up just in time to disconnect the Doctor. Disoriented and synaesthetic, and confused by the strange memories which the processor has caused to surface in his mind, the Doctor offers no resistance as the figure carries him back through the transmat to Graystairs. Meanwhile, Sooal reports to his masters at Graystairs, telling them that his work is nearly complete; but the figures are impatient, and angered by their reliance on this inferior life form.

Michael finally works up the nerve to contact his mother, who’s happy to see him here until he admits that he’s gone AWOL. He’s had enough of UNIT, and he wants out. Nevertheless, knowing that something’s wrong at Graystairs, he accompanies Joyce back to the clinic to rescue his grandmother. While Joyce goes in through the front door, he breaks in through the back, just in time to see Ace emerge from the cellar, carrying the disoriented Doctor. Sooal then arrives and tries to kill them, and although he knows that the man Ace is carrying must be the Doctor, Michael nevertheless helps her to carry him out to the woods. They are pursued by two figures from Graystairs, armed with pulse rifles, and as the Doctor is slowing them down they’re forced to leave him in a nearby cottage and lure the pursuit away. They succeed, and on their way back, Ace tells Michael that she’s just spent 24 hours in the Orkney Islands -- and that she travelled through time to get back here. But when they return to the cottage, there’s no sign of the Doctor. While Ace searches the village, Michael returns to Graystairs, where he finds his mother sitting up with her own; Norma has had a relapse, and Joyce, ashamed of lashing out at her in frustration, has decided to stay here, despite the danger, to ensure that she gets the treatment she desperately needs.

Meanwhile, a younger version of Ace is still stuck in the Orkneys with an angry young student who can’t accept the story she’s telling him. Alexander offers to take her to their campsite on the nearest island in order to get out of John’s hair for a while, but when they arrive they meet a strange middle-aged couple with a black dog -- apparently the same couple Ace met in Muirbridge less than a day ago. Hoping that the couple also owns a transmat, Ace and Alexander break into their cottage, and find evidence that the couple are indeed alien. When they return to their campsite, however, they find that their radio has been smashed. Ace sends Alexander back to the boat to warn John, but she remains on the island, unwilling to draw the aliens’ attention to the brothers. Moments after Alexander goes, however, Megan shows herself and prepares to kill Ace, revealing that she already left the Doctor dying in the spaceship. Ace manages to distract Megan and flee, but the island is too small for her to hide from both Megan and the middle-aged couple for very long...

The Doctor is with Eddie, who saw him fleeing from Graystairs and wants to know what’s happened to him. He believes that the people at Graystairs have put false memories of war in his mind, and he’s frightened of how much he loves the visions of death and torture. The Doctor, slowly recovering from his experiences, returns to the pub with Eddie, where he meets Ace; however, he still doesn’t know who “Stacy Chambers” is, and Ace can’t tell him everything that’s happened to her, as some things haven’t actually happened yet. Michael then arrives, and when he sees that the Doctor has apparently recovered, he punches him in the face. When the other soldiers at UNIT learned that Michael’s mother and father, Dr Joyce Brunner and General Terrance Ashworth, were friends of the Doctor, they hazed him unmercifully, blaming the Doctor for strutting around arrogantly and thinking himself so clever while the ordinary troops laid down their lives fighting aliens on the front lines. At first, Michael tried defending the Doctor’s reputation, until his one friend, Andy, was killed during the Talichre invasion; now he just wants out. But now he faces a court-martial if he returns, Ace is infuriated with him, and when he looks at the Doctor all he sees is a confused man with a bloody nose.

The pub is then attacked by some of the residents from Graystairs, who no longer seem confused and disoriented -- rather, they’re armed with pulse rifles, and determined to get Eddie back. Ace uses her nitro-nine to hold off the attackers, but Michael realises he’s fighting aliens to save the Doctor and walks away, leaving the others to fend for themselves. The Doctor, Ace, Claire and Eddie shelter in the local church, where the Doctor finally realises that Eddie’s memories of war are his own memories after all. Although afraid of what he might become, Eddie allows the Doctor to use a telepathic connection to fully restore his past life to him -- and then walks out to join his fellow Tulk. He can no longer be considered “Eddie”; once again, he is Onaaka Matrin, commander of the Second Tulkan War Fleet.

The Doctor sends the confused Claire home while he and Ace follow the Tulk back to Graystairs, but on the way, they run into the middle-aged couple and their dog. Ace already knows that they’re not to be trusted, and when the Doctor recognises the “dog” as a Landine, a genetically engineered guard animal, he realises that they are Annarene, wearing fleshsuits to disguise themselves as humans. The couple explain that the Tulkan Empire attacked the Annarene four years ago, but the Protectorate successfully defeated the Tulk and sentenced their War Council to memory erasure and exile on a penal world. For some reason, Sooal rescued the War Council and took them to Earth to restore their memories under the pretense of conducting Alzheimer’s research; now his work is complete. The Doctor, Ace and the Annarene return to Graystairs, where they overhear the Tulk giving Sooal fragments of the access code for the silver dome; each member of the War Council remembers one part of the code. The Doctor realises that he’s been on the wrong trail all along; Norma didn’t overhear a comment about “Stacy Chambers”, but a “stasis chamber”, a sealed bubble of space-time containing the weapons with which the Tulk intend to conquer the Earth and make it the centre of the new Tulkan Empire.

The Tulk finish giving Sooal their codes, and before the Doctor can do anything, Sooal guns them all down. The Annarene then reveal themselves, and as the Landine grabs away Sooal’s gun, the Annarene reveal that they’re not here to arrest him at all. As the Doctor suspected when he realised that they’d been watching Sooal for years without making a move, they’re really after the weapons in the stasis chamber; they belong to a splinter faction which intends to restore the warlie past of the Annarene, and after surreptitiously helping Sooal to rescue the Tulk, their faction sent two teams to Earth to monitor his progress. Ace realises that the Annarene in the Orkneys are responsible for attacking John and Alexander’s boat and stealing the control sphere -- but she inadvertently blurts out this accusation before realising that this hasn’t happened yet, thus revealing the location of the missing sphere to the Annarene.

Back in the Orkneys, Megan finally corners Ace, but the Annarene male then arrives and thanks Ace for telling them where to find the control sphere; the Landine has already retrieved it from the boat, driving the frightened brothers overboard. Ace doesn’t know what he’s talking about, but Megan realises that all of her work and waiting has been for nothing and shoots the Annarene in a fit of rage. This team’s Landine attacks her, and both fall over the edge of the cliff to their deaths. John and Alexander have survived, but now the surviving Annarene from this team has the control sphere, and she takes it to Muirbridge. There, the other Annarene have locked the Doctor, Sooal and the older Ace in the cellar. The Doctor has deduced from Sooal’s appearance that he’s suffering from progeria -- premature aging, a genetic condition engineered into his people by the Tulk. As well as weapons, the stasis chamber contains a metabolic stabiliser which the Tulk planned to use to secure their own immortality, and which can cure Sooal’s condition -- if he can get to it. The processing array on the spaceship was a backup system, which Sooal was using to calculate the frequency of the stasis chamber in case he failed to recover the Tulk’s memories. Having confirmed his suspicions, the Doctor seems to retreat into himself; Ace fears that he’s suffering a relapse, but upstairs, Connie and Jessie hear a voice in their heads, calling their names...

Joyce has overheard the slaughter of the Tulk and the Annarene’s revelations, and realises that she can no longer risk keeping her mother here. Michael returns and helps her to get Norma to the B&B, but there, they see the Annarene ship arriving. Despite himself, Michael returns to Graystairs, where he subdues the Landine and rescues the Doctor, Ace and Sooal. The Doctor takes them to the transmat, but the Annarene apparently stop him before he can deactivate it; however, before they can take the control sphere through, Connie and Jessie arrive and claim that they are true agents of the Annarene Protectorate, here to arrest the renegades. In the ensuing confusion, Michael overpowers his guard, and the Doctor grabs the control sphere. The Annarene and Sooal flee through the transmat, and discover too late that the Doctor had already reprogrammed it, using the frequency he calculated while connected to the processing array. The villains have just transported themselves directly into the stasis chamber, where they will be frozen in Time until it’s turned off.

Back in the Orkneys, Alexander returns to the island to check on Ace, who gives him a postcard for the Doctor containing the exact co-ordinates, time and date and asking for rescue. The TARDIS shows up on schedule, but when Ace enters, the Doctor seems distracted and demands to know exactly when they first arrived in Muirbridge. When Ace sees that he’s blocked the door and hears somebody pounding on the other side, she realises that this is the same encounter she experienced before their arrival -- but this time, from the other side. Joyce didn’t leave the time and date in her letter, and the postmark was smudged; but as the mail also contained Ace’s letter, posted from the same general area, the Doctor picked her up first in order to find out where to go next. Ace tells the Doctor when and where they first arrived, but then recalls Megan’s claim that she’d left the Doctor dying in the spaceship -- and thus, while the Doctor is trying to calm down the Ace on the other side of the door, this Ace changes the co-ordinates on the TARDIS console. Without realising it, the Doctor drops her off in Muirbridge two days too early -- just moments before his “first” arrival. He’s furious when he learns what she’s done, and says as much when they meet again outside the TARDIS, but by that point it’s too late for him to do anything about it. Over the course of the next day, Ace hides from her younger self, makes a bacon sandwich in the cellar kitchen and hits Megan with a frying pan to save her younger self from discovery; and eventually, the time comes for her to emerge and rescue the Doctor from the processing array...