On the planet Caresh, the important work of a Time Lord named Roche is interrupted by Magus Amathon, a representative of the Vortex Dwellers. Little is known of these beings, and though Amathon informs Roche that his work threatens the Dwellers’ Realm, Roche refuses to abandon his important work without proof of the threat. Amathon vanishes as a sun watcher arrives from Dassar College, but as she begins to deliver her report, two leathery eggs appear in Roche’s Citadel and hatch, emitting a blinding light...
Israel, 1972: the Doctor and Jo trace a temporal distortion to an archaeological expedition led by Professor Ezekiel Child, who believes that he’s found evidence in the Dead Sea Scrolls that an alien spacecraft once crashed near Masada. However, there’s a strong possibility that he’s mistaken -- and although he and the Doctor do find something buried beneath the sands, it’s not what either of them expected.
London, 1999: science fiction fan Simon Haldane meets a strange woman on the street, and soon realizes that she’s not of this Earth. Troy Game, for her part, can’t remember how she came to be here, and initially believes herself to be on a strange island on her homeworld -- but this belief doesn’t last long. The people here have long hair and “earlobes”, and their behaviour is entirely alien to her; also, she weighs less than she should, and there is only one sun. Her natural gift of language-telepathy has altered, enabling her to pick up the thoughts and emotions of the strangers around her. Shaken, she finally realizes that she’s farther from home than she’d dreamed possible, and Simon, deeply attracted to her -- both physically and by what she represents -- allows her to stay in his flat and tries to help her find out how she came to be on Earth.
Not far away, Michael Sheridan, a reporter for Open Minds, questions college student Helen Ayre about the disappearance of her co-worker James Preedy. Last night, James vanished while cleaning rooms at the Regis Seaview Hotel, and outside Room 18, Helen found a statue, a perfect granite replica of Preedy with an expression of terror frozen on his face. Apparently, a man collapsed in the hotel earlier that day after an argument with a young female friend, and was taken to hospital; however, Helen doesn’t know anything more about the incident, and she clams up when she realizes that Sheridan works for a tabloid which features stories of the paranormal and alien abductions.
The Doctor’s freedom has been restored, but his failure to locate Metebelis 3 indicates that the TARDIS navigational system needs recalibrating. He thus takes Jo on a tour of landmark worlds, but the tour is interrupted on the planet Dagusa by an old acquaintance, the Time Lady Solenti. Solenti -- who is blind, and is guided by a telepathic bond with her seeing-eye dog, Jess -- explains that she needs the Doctor’s unofficial help, as her former partner Lord Roche has vanished and has somehow become involved in the creation of a time fracture. The Doctor detected the fracture in 1972, although when he traced it to Israel the “alien spacecraft” he believed to be the source turned out to be a Spitfire which had crashed in 1948, thus ending the unfortunate Professor Child’s career. Relying on the Doctor’s natural curiosity, Solenti gives him a navigational device which will direct him to the fracture two days before it opens. The Doctor reluctantly agrees to help, and watches Solenti depart in her TARDIS -- which is equipped with an “atrium circuit”, which affects the psychology of the observer and makes the TARDIS appear to be a perfectly natural part of the environment.
Simon finds living with Troy Game a frustrating experience; her culture has no taboos on nudity or any concept of “personal space”, but she shows no sexual interest in him. Over the next two weeks her memories begin to return, but while she remembers the face of a man involved in her arrival on Earth, she can’t remember his name. She has a vision of the number “18”, though the meaning of the symbol eludes her. Simon uses a computer program to simulate the binary system which Troy Game described, and finds that Caresh’s orbit resembles the number “8” -- at least until it settles into a stable orbit around the smaller sun, Ember, resulting in a permanent ice age. If the projection is accurate, Troy Game’s civilisation is doomed.
Simon gives up and contacts Michael Sheridan for help while on his break at work; they once had a bitter argument over the lack of real science in Sheridan’s articles, but Sheridan is the only man Simon knows who might have a lead on what happened to Troy Game. However, when Simon contacts him Sheridan mentions the petrified man from the hotel, and Simon hangs up, afraid that Troy Game might be in danger -- or, worse, responsible. He is unable to go straight home; due to his concerns for Troy Game, he’s been letting his attention slip at work, and he must attend a retirement party for a co-worker for fear of alienating his co-workers and losing his job. At the party, he drinks too much, and when his co-worker Amelia makes a pass at him he rejects her, realizing that he’s in love with Troy Game. He returns to his flat just as Troy Game steps out of the shower, and foolishly and drunkenly tries to “assert” himself. When he wakes up hours later, he has a cracked rib and a bruised eye, and Troy Game, finally realizing that the people of this world do not go through periodic fertility cycles, has fled.
Solenti’s navigational device is programmed to deliver the Doctor two days before the start of the fracture, but instead of arriving in Masada, the TARDIS materializes in England -- and since the device had synchronized the TARDIS’ materialization to the speed of Earth’s rotation at Israel’s latitude, the ship hurtles across the countryside, leaving a trail of devastation before the Doctor realizes what the problem is and corrects it, bringing the TARDIS to a halt in a car park in Chichester. But his problems are just beginning. The fracture proves to be contratemporal, stretching back in time rather than forwards, which is why they materialized here, in 1999... and since they materialized “before” it opened, they’ve been navigating within the fracture itself, throwing them further off course. Instead of two days, they only have a few hours to find the cause of the fracture.
According to Solenti’s tracker, the fracture has one primary and two secondary sources -- but the primary source is moving, and the secondary sources only show up on a live feed, not on playback. As time is short, the Doctor sends Jo to investigate the nearby secondary source while he investigates the primary source. He must rely on the TARDIS to get him there on time, but on the way back, he runs into Michael Sheridan, who has come to Chichester in response to Simon’s call. Sheridan has missed meeting Simon, who is out on the streets searching for Troy Game in an attempt to make amends for his behaviour; however, he nearly gets the scoop of his life when he sees the TARDIS and recognizes it from underground rumours. The Doctor spots him taking pictures, and before dematerializing he operates a device on the TARDIS console, erasing the data in Sheridan’s digital camera and his laptop computer.
On her way to the secondary source, Jo passes Troy Game busking in an Underground station. Her journey seems to take a long time, as if some side-effect of the fracture is pushing her back, but she eventually reaches her destination, a mental hospital just outside town. There, she finds that the resistance is emanating from a certain room, but when she asks the man inside if he is Lord Roche, a creature like a snake-headed jackal appears from nowhere and bears down on him. Roche pushes Jo back, not just physically but through Time as well, and she finds herself back in Chichester a half-hour previously. This time, when she passes the Underground station, Troy Game sees an image of Lord Roche in Jo’s mind, recognizes him from her dreams, and accompanies Jo to the hospital.
In a nature reserve in Westbourne Forest, a lake has just drained away through a buried set of doors. Student Zeke Child and his friends Ben and Patric Keller investigate, theorizing that the doors were sealed for centuries by the weight of the water, until a combination of the hot summer and vibrations from the motorway caused the seal to crack. The caves beyond the doors seem larger than could be natural -- and there’s something else in the “caves”, a creature like a snake-headed jackal which has nearly starved to death but can now sense time energy pouring in through the open doors. Ben and Patric encounter the creature, which turns them to stone; however, Zeke is rescued by the Doctor, who saw him entering the caves and recognized him as the middle-aged man he met twenty years ago. Back in 1972, the Doctor’s detector did lead him to the source of the time fracture after all -- but it was the archaeologist all along, not the target of his expedition. Zeke Child is the primary source of the fracture.
The Doctor recognizes the “caves” as the petrified remains of a TARDIS, presumably Roche’s, and Zeke finds the petrified body of another snake-jackal creature which the Doctor identifies as a Fury -- one of the fracture’s secondary sources. These are beasts which dwell in the Vortex, and the Doctor assumes that they were drawn to Earth by the time fracture. The surviving Fury gorges itself on the energy flowing into the dead TARDIS and explodes, and the stone TARDIS crumbles around it. The Doctor escapes in time, but Zeke is caught by the ensuing temporal blast and knocked unconscious. The Doctor takes him back to the TARDIS and sedates him, realizing that he’s now suffering from Jeapes’ Syndrome, which will kick in when he awakens. He will live the next 27 years of his life backwards, and although history will accommodate the anomaly, his life will become a series of disappointments as he fails to have any impact on the world around him. The strain on space-time will also open the time fracture which the Doctor detected. However, since Zeke’s accident happened as a result of Lord Roche sending his TARDIS back in Time with two Furies trapped inside, then if the Doctor can prevent him from doing so, he can prevent the fracture from occurring.
Jo and Troy Game reach the hospital, where Jo sees the shed skin of the Hunter Fury outside Lord Roche’s room; when she touches it, it crumbles in her hand. Lord Roche, acting entirely on instinct to hide his conscious mind from the hunting Fury, hypnotises Troy Game and plants instructions in her mind and a key to room 18 of the Seaview Hotel in her hand. As Jo and Troy Game head for the hotel, Zeke Child is exposed to the temporal blast, and the time fracture thus “opens” -- and since it stretches backwards, this means that it no longer exists. Roche had been hiding his mindscent in the background “noise” of the fracture, and since this is no longer possible, he must use another Time Lord talent to evade the Furies. Albeit reluctantly, he swaps his mindscent with that of hospital orderly Donald MacRae; physically, they remain unchanged, but anybody looking at one of them will perceive the other. Roche then slips out of the hospital, while the Hunter Fury targets and petrifies Donald. Roche again swaps mindscent with the person nearest to him when Donald dies, and continues to do the same each time the Fury strikes. Thus leaving a trail of the innocent dead behind him -- including Simon Haldane, who is unlucky enough to run into Roche while looking for Troy Game -- Roche reaches the hotel and waits outside for Troy Game to bring his TARDIS to him.
Following Roche’s instructions, Troy Game enters room 18 of the Seaview Hotel and opens up its shower cubicle to reveal that this is Roche’s TARDIS. However, the second Fury, a Sentry, is standing guard, and when it attacks, Troy Game accidentally locks Jo out of the TARDIS. Trying to get in, Jo touches the TARDIS with the hand which brushed against the shed skin outside the hospital, and the TARDIS’ Hostile Action Displacement System interprets this as an attack and dematerializes. Fortunately, Room 18 proves to be an extension of the atrium circuit, and as it vanishes, Jo is expelled into the corridor outside before the Sentry Fury can kill her.
Roche’s TARDIS materializes outside in the shape of an ice cream kiosk, and Roche runs for it -- but, exhausted and disoriented by his exertions, he panics when Troy Game steps out of the TARDIS and calls him by name, and instinctively steps back into the path of an oncoming bus. The Doctor disembarks from the bus, realizes what’s happened, and covers Roche with his coat so nobody will see him regenerate; however, some confusion ensues when Roche regenerates into a double of the Doctor. Now suffering from post-regenerative disorientation, Roche passes out, and the Doctor uses Roche’s stolen car to return to his own TARDIS with Roche, Troy Game, and Jo. There, he places Roche in the Zero Room to recover, believing that he’s solved the problem of the time fracture. Unaware that the Furies were sent to assassinate Roche by the Vortex Dwellers, he believes they were attracted by the time fracture which would have been caused had Roche used his TARDIS to dispose of them in the past; by preventing Roche from doing so, he has prevented this paradox.
The Doctor’s theory seems borne out when the confused Ezekiel Child enters the console room, obviously living forward through Time rather than backward. The Doctor agrees to take him home, but as they pass the hotel, the Doctor sees the Furies still standing guard by the ice cream kiosk -- and “Child” is revealed to be Roche, who swapped mindscents again in order to trick the Doctor. The real Zeke Child finally does wake, and vanishes, his timeline going into reverse as the Doctor had predicted. However, the Furies are confused by the proximity of two physically similar Time Lords, and erroneously attack the Doctor, giving Roche time enough to get to his TARDIS before they realize their mistake and break off the attack. Roche pilots his TARDIS to a field where a large crowd of people is waiting to watch a solar eclipse, timing things so that the Furies attack just as the sun is blotted out. The sense of awe emanating from so many people at the same time momentarily overwhelms the Furies, long enough for Roche to get clear while his TARDIS dematerializes, sweeping up the Furies in its atrium circuit and transporting them 1,300 years back in Time. Trapped inside, cut off from the Vortex, they will eventually starve to death -- or so Roche believes. Now he must figure out how to escape from Earth, without a TARDIS, while resembling the Third Doctor.
Disheartened by his failure, the Doctor returns to the TARDIS and pilots Troy Game back to Caresh. Her memories are returning as her telepathic “gift” fades, and the Doctor realizes that this is a result of her trip in Roche’s TARDIS. When the Furies attacked Roche, Troy Game fled into his TARDIS with him. His TARDIS thus attempted to give her the gift of language-telepathy, but as her species already possesses this gift naturally, the interference opened up other telepathic centres of her mind and blocked her recent memories. She doesn’t know exactly what Roche was doing, but believes it has something to do with a neutron star which has been approaching Caresh. Is Roche trying to manipulate the star’s course? If so, why?
Elsewhere, Solenti and Jess are contacted by Magus Amathon, who is this time acting without the approval of the Curia of Nineteen and will be punished for it. He reveals that by altering the course of the neutron star, Roche threatened to destroy the Realm, which is why the Curia unleashed the Furies on him. However, the Realm is still in danger. Solenti, knowing that Roche would leave well enough alone after barely surviving a Fury attack, realizes that the well-intentioned Doctor is going to finish Roche’s work for him...
The TARDIS materializes on Caresh, but since the navigational system still needs alignment, it does not appear near Roche’s Citadel; rather, it materializes on Stakisha, near Fell Island, two islands which are hostile to Dassar and to those who follow Lord Roche. When Lord Roche came to Caresh, he promised to end the cold years, but while he works he has suppressed the natives’ own technological advances and innovations, leaving them less capable of surviving the cold years when they do occur. The Doctor, Jo and Troy Game are arrested by Rak Toos, who initially believes the Doctor to be Lord Roche himself; however, he is puzzled by the Doctor’s responses to his questions, and by the fact that the Doctor is not carrying Roche’s “mercy gun”. Unsure what to do, he decides to send his captives to Fell for interrogation.
The Doctor recovers his sonic screwdriver from the sympathetic Rak Shal, but before he can use it to escape he and his friends are loaded aboard an electric train bound for the causeway to Fell. The Doctor opens the carriage door and climbs onto the train roof, planning to knock free the power cable, slow the train just long enough for Jo and Troy Game to leap to safety, and then replace the cable before the train’s crew realize what’s happening. The plan fails when the cable leaps free prematurely, and Troy Game jumps at the wrong time, breaking her leg when she lands. The Doctor and Jo get her to the harbours, where they are lucky enough to meet a native of Dassar, Troy Sheltek. She helps them aboard the Black Flower, a ship bound for Dassar, and once on board, Troy Game sleeps for a day and a half, her injury healing. As she recovers, she asks the Doctor about the “mercy gun” which Rak Toos had mentioned, and the Doctor identifies it as a Time Lord weapon; the first shot stuns the victim, and the second kills, the theory being that if someone must be shot twice they haven’t learned their lesson. The Doctor is amused to learn that Roche himself was once shot with his own gun by the hostile natives of Fell Island.
The ship is attacked by Leshe, giant carnivorous insects which are carried from the poles by icebergs and glaciers in the cold years, wreaking havoc on the inhabited lands of Caresh as they go. During the battle, the Doctor is knocked overboard, but he is rescued by Solenti, who has come to tell the Doctor to go away. She explains that she and Roche first came to Caresh on behalf of the Time Lords to shut down a temporal projector which the natives had invented and which allowed them to foresee whether the next several years would be warm or cold. Roche became fascinated with the planet for some reason, and decided to help the natives adapt their own technology to alter the course of the passing neutron star, thus nudging Caresh into a stable orbit around the warm sun, Beacon. Unfortunately, Roche had to leave before his work was complete -- and the tidal effects of the neutron star will render Caresh uninhabitable. The Doctor and Solenti are attacked by Leshe before Solenti can explain why the Doctor must not interfere further, and Jess is mortally injured saving Solenti’s life. The grieving Solenti takes the dying dog into her TARDIS and leaves, and the Doctor, assuming that Solenti’s warning was just more bureaucratic Time Lord nonsense, decides to save Caresh and worry about the consequences later.
The Doctor returns to the Black Flower and journeys on to the Citadel, but they are pursued by soldiers from Fell Island attempting to recapture the escaping prisoners. Troy Game opens up the Citadel and locks the soldiers out, and then collapses on a couch to recuperate from her exertions. The Doctor studies the controls of the stellar manipulator, and determines that the figure “18” which Troy Game dreamt represents the straight path of the neutron star passing the double-loop orbit of Caresh around its two suns. He also learns that the final adjustment to the neutron star must be made in exactly 63 minutes; otherwise, Caresh is doomed.
Sleeping on the couch, Troy Game inadvertently accesses Roche’s telepathic diary, and learns that he discovered a civilisation on the continent of Fayon, which has not yet made contact with the Archipelago. What Roche found there apparently inspired his decision to “correct” Caresh’s orbit, but he recently learned that he’d made a mistake; although Caresh’s orbit will indeed be altered as he intended, he had forgotten to take into account the searchlights of radiation emitted from the neutron star’s poles. One of the searchlights will sweep across the Archipelago as the neutron star passes, and although the Fayoni will be saved -- which is all Roche really cares about -- all life in Troy Game’s homelands will be wiped out.
Troy Game wakes to find the Doctor making the final adjustments, and concludes that Lord Roche must have overpowered the real Doctor, returned to the TARDIS and tried to trick her and Jo. Before the Doctor or Jo can stop her, she opens the Citadel door, and the soldiers from Fell, led by Rak Toos, enter and arrest them all. Outside, the sky is shrouded with clouds, and the approaching neutron star cannot be seen. The soldiers thus drag the protesting Doctor away to face judgement, leaving Jo and Troy Game in the Citadel as hostages. Jo manages to convince Troy Game that she’s wrong; if the radiation searchlight sweeps across the Archipelago then even the inhabitants of the Citadel would be placed at risk, and Lord Roche would never risk his own life even for the sake of a civilisation.
Troy Game, realizing that she may be right, helps Jo to overpower the guards -- and then shoots the Doctor with Lord Roche’s mercy gun, which she had found in the Citadel. The Doctor is only stunned rather than killed, thus proving his identity, and the clouds then part, revealing the approaching neutron star and convincing Rak Toos that the Doctor was telling the truth all along. Unfortunately, the Doctor remains paralysed for hours, missing the deadline for the final adjustments. It seems that Caresh is doomed, but just in time, he realizes that he and Lord Roche have both overlooked an obvious alternative -- rather than nudging Caresh into orbit around Beacon, the Doctor can nudge it into a closer orbit around the “colder” star, Ember, with the same results. Fortunately, he completes his calculations in time and completes the adjustment successfully, saving the planet and granting it a stable, warmer climate.
Rak Toos agrees to transport the Doctor back to his TARDIS, and before they leave, Solenti -- now accompanied by a “new” dog named Jess -- contacts him and informs him that the beings who sent the Furies no longer feel threatened. The Realm has survived intact, and although Magus Amathon revealed its secrets to Solenti, his sentence is commuted to exile. The Curia calls off their Furies, petrifying them in stone moments before Lord Roche lures them into his trap; thus, he has no need to send his TARDIS back in Time, and is able to leave Earth, although he still resembles the Third Doctor. Since Roche’s TARDIS never goes back in Time, Zeke Child never encounters it, the time fracture does not occur, and Zeke lives out an ordinary, fruitful life forwards through Time. On Caresh, the Doctor and Jo set off to investigate the Fayoni and find out why Roche felt it was worth wiping out the Archipelago to save them, while Troy Game, whose homeland has been devastated by the Leshe during her absence, swims into the sea and grows gills, becoming a sea-dweller and finally feeling at home.