Several years ago, a young woman was found brutally murdered in the depths of Norswood. Now, late at night, a group of students from the local university have decided to hold a séance to contact the spirit of the murdered girl. Ralph, Meg and Tommy all regard this as a lark, but Joan Cox insists that they take their attempt seriously, and Chloe Pennick seems particularly affected by the grim atmosphere in the woods. As the séance begins, she, and she alone, hears an evil voice screaming her name, and a wild-faced demon which only she can see bursts out of the shadows and physically attacks her, screaming abuse at her disobedience. Chloe flees in panic, only to awaken back in her dormitory bed -- but she and her friends really did hold a séance the night before, and when she throws off her bedsheet she finds her body covered in scratches and bruises from her flight...

As always, the TARDIS chooses a destination by collapsing probabilities into inevitability and materialising in the resultant timeline; but this time, it’s changed course by itself, and the Doctor doesn’t know why. There appears to be a small anomaly in the multiverse, and the Doctor sets the TARDIS probability compensators to monitor and correct it while he and Leela emerge to investigate. The TARDIS has materialised in Norswood, and Leela also feels oppressed by the shadows of the wood. The Doctor dismisses this as a side-effect of her warrior training, but although the feeling fades as they leave the woods, it begins to grow stronger again as she approaches the University, as though she can sense something dangerous watching her...

At the University, parapsychologist Barry Hitchins’ experiments into psi powers have irritated his fellow academics; physicist John Finer in particular has made no secret of his anger at the funding of this pseudo-science. Hitchins was something of a media hound until an anonymous note from the head of the Kellerfield Research Foundation threatened to cut off his funding, at which point he began concentrating on proper academic research rather than diverting the funds for his glory-seeking ghostbusting activities. To his surprise, he found that many students at the University exhibited higher than average paranormal powers, and he’s selected six of the most promising for further tests. Their “powers” seem to have vanished since the first wave of tests, but he perseveres; however, his attempt to conduct a Zener card test with Josh Randall and Tommy Carmodie is a dismal failure. Josh, once the most promising of his subjects, now seems to be taking the piss out of the entire process, giving a sarcastic play-by-play without identifying a single card correctly. Barry calls an end to the experiment, and as Josh and Tommy retreat to the pub, Finer pops by to confront Barry, claiming that his experiments are distracting Josh from his worthwhile studies in the engineering department. If Barry gives him the slightest excuse, Finer will see him fired.

Barry’s students discuss Chloe’s hysterical outburst during the séance, and Josh irritates both Joan and Chloe by his contempt for their beliefs; he insists that the supernatural is nonsense, and deliberately calls Chloe “Clarry” just to irritate her. Moments later, Chloe hears the voice of the demon screaming the name “Clarry” at her, and flees from the pub in a panic only to run into the Doctor and Leela outside. Assuming that the eccentric pair are members of an improv theatre group, she feels calmed by their presence and offers to arrange guest quarters for them. The Doctor doesn’t take her claims to be haunted seriously until she reveals that there’s a parapsychology department on the campus; he then realises that he’s been babbling to Chloe, telling her that he and Leela are time travellers. She obviously thinks he’s just been talking nonsense to cheer her up, but Leela is angry; the Doctor has always told her to be reticent about her origins, and she feels he’s betrayed her trust. As the Doctor realises that he’s been behaving irrationally, he sees the evening shadows shifting in the corner of his eyes and hears the voice of the demon screaming abuse at him. Or perhaps it’s just his imagination.

Joan convinces her fellow students to hold another séance in her dormitory room; she wants to believe in the existence of another state of reality, and is angry when Josh claims he just came along to watch the others make fools of themselves. Nevertheless, he watches with great interest as the Ouija board’s planchette begins to move by itself. The planchette claims that Joan will be the next to die and then flips off the board as the room’s halogen lamps explode. As the seriously shaken Joan retreats, Josh insists that this was all in their heads; he claims to have taken the initial tests for a laugh, and thinks Barry a fool for calling him gifted.

The Doctor realises that if there is a multiverse field weakness in the vicinity of the psi experiments, perhaps they’re feeding into it and causing reality to become unstable. Or the weakness may be enhancing the psi experiments, causing those with vulnerable minds to succumb to delusions. He and Leela are also being affected; the Doctor actually sleeps all night for once, while Leela recalls the Doctor’s claim that her tribe’s enemies used psi powers and thus concludes that there’s a Tesh somewhere on the campus. The next morning, the Doctor leaves her alone while he speaks to Barry, and Leela, already angry with him for explaining himself to strangers while leaving her in the dark, fears that he’s going to betray her to the Tesh. She thus refuses to use cutlery while eating her breakfast, in order to assure herself that she’s still a Sevateem warrior. When one of the students in the cafeteria mocks her, she attacks him and fights her way past the security guards, intending to wait for the Doctor at the TARDIS. As she enters Norswood she sees a local man named Frank fleeing from the woods, having been driven out by an inexplicable feeling of panic. She too begins to feel tense, and when she finds the TARDIS missing she fears that the Doctor has abandoned her to the Tesh. A strange black void then appears in the forest clearing and draws her towards it, and she seems to see and hear the demon screaming abuse at her; but she realises that she is feeling someone else’s fears, not her own. She resists the pull of the void, which folds up into itself and vanishes to reveal the TARDIS still standing in the clearing.

The Doctor picks up a free sample of Clearspring bottled water on his way to the parapsychology department. Barry isn’t there, but the Doctor meets philosophy professor Bill Parnaby, one of the few people on campus who counts himself as both Barry’s friend and Finer’s. The Doctor accompanies Bill to his office, but when Parnaby summons security guard Len Spark, the Doctor realises that he’s been babbling again and has nearly convinced Parnaby that he’s mad. He thus begins to wonder whether the bottled water has been spiked. He tries to stay calm and enlist the help of the campus security chief, Fred, but since Leela beat up Fred on her way out, Fred calls in his brother-in-law, Detective Sergeant Bill Simpson, to arrest the Doctor on suspicion of industrial espionage. Simpson doesn’t like Fred very much, and releases the Doctor after running a standard background check and ordering him to stay out of trouble. The Doctor heads back to the university, and on the way, he sees a truck with the Clearwater logo; the driver isn’t actually a Clearwater employee, but he does know that the water is bottled in Yorkshire, distributed from Birmingham, and sold only in this particular area. The driver assumes this to be part of a tax dodge, but the Doctor fears that there may be a more sinister explanation -- unless the multiverse overlap is causing him to become paranoid.

Barry sets up an experiment in which one student in a cubicle sends mental images to another student in a sensory-deprivation tank. He hopes to get Josh into the tank, but Josh refuses, repeatedly calling it a “bloody great coffin”. Barry tries to manipulate him into volunteering by threatening to drop him from the experiments at Finer’s request, but while Josh is angered by someone else’s attempt to dictate what he can do, he unexpectedly volunteers Chloe instead of himself just as Barry thought he’d won. Clearly Josh was aware of what Barry was trying to do, and was just toying with him. Chloe volunteers of her own accord when she arrives, as her encounter with the Doctor and Leela has cheered her considerably. However, things start to go wrong almost immediately; Chloe seems to fall asleep within moments of entering the tank, and nobody on the outside realises that she’s having a panic attack, convinced that she’s drowning in a pool of blood and that nobody can hear her screaming for help. Tommy unexpectedly claims to be thinking of breakfast, which is one of the images on the photographs Barry had given to Josh. Meg claims to see blood pouring out of the tank, and when she and Tommy rush over to investigate they slip in the blood and knock themselves out on the floor. Josh seems to have fallen asleep in his cubicle, but he no longer appears to be in the same cubicle he entered when the experiment began. Convinced that his students are playing a joke on him, Barry tries to open the tank himself, only to find that the lid is impossibly jammed shut -- and he then collapses himself, as blood begins to pour out of his body...

Leela returns to the university, seeking to warn the Doctor about the void in the woods. She arrives at the parapsychology department to find bodies lying around a room and blood covering the floor and walls -- and a darkness in the corner like the void from the woods. As before, the void folds up into itself and vanishes, and she then realises that the liquid she’s seeing is ordinary water with a slight red tinge. It’s not as thick or as prevalent as it seemed when she first stepped into the room, so why did she think it was? The students begin to recover, and as Josh stirs and steps out of his cubicle, Leela levers the unconscious Barry’s body off the tank, freeing Chloe. The tank couldn’t have jammed shut, as there are no catches to jam. Chloe believes that Leela has saved her life, but Josh, snide as ever, asks what she was being saved from -- and seems to expect an honest answer.

Simpson and his associate, Detective Constable Martin Bartok, are calle back to the University on a more serious matter; Joan Cox has committed suicide. Fred seems to expect Simpson to help him keep it quiet, but Simpson becomes angry when Bartok discovers that Joan was involved in occult studies and parapsychology experiments and that Fred deliberately didn’t tell him. Fred is hoping to use his family connections to avoid publicity, but Simpson threatens to arrest him if he holds back anything else. Meanwhile, Barry’s other students are shocked to realise that Joan was slashing her wrists at the very moment all of them were “dreaming” of blood in Barry’s laboratory. Josh scoffs at their credulity and claims that Joan faked the Ouija board incident herself to add theatricality to her suicide. Chloe tries to show sympathy, but she’s having trouble thinking clearly, as suddenly it seems that the others are speaking her thoughts aloud mere moments after she thinks them. She thus offers no resistance when Josh cruelly suggests holding another séance, at the local cemetery.

The Doctor returns to the university, aware that the field effect may be generating paranoia in vulnerable minds but unaware that he’s become fixated about getting the bottled water tested. When he arrives at Barry’s laboratory, he ignores Leela’s attempts to speak to him, and instead questions Barry about his experiments. Barry, assuming that this anonymous Doctor is an agent for the Kellerfield foundation, tells him everything; he has concluded that a power surge wiped the tapes of the experiment and caused the insulation gel in the tank to leak into the water, producing toxic fumes which caused everyone in the laboratory to hallucinate. Unfortunately for his theory, no combustion took place and the gel is non-toxic. The Doctor finally listens to Leela, and realises that the darkness she is describing is an interdimensional void; if they are becoming more prevalent, the weakness may be more serious than he’d thought, and reality itself may be on the verge of collapse.

That night, the surviving students break into the local cemetery with a Ouija board, but while Josh eggs the others on, he sits out the séance himself so they can’t accuse him of moving the planchette around. Chloe begins to feel once again that the others are speaking her thoughts aloud, but it could just be her imagination, and the shrouded figure she sees watching her from the shadows may be a trick of the light. When the Ouija board spells out a threat directed at “Clarry”, she remembers the demon screaming this name at her and realises that she will be the next to die; however, the others assume that the message refers to the late Clarissa Johnson, whose tombstone rests nearby. As the others pack up and leave, Josh angrily mutters “Clarry” under his breath, but claims that he isn’t talking to Chloe; he believes that the tombstone was a coincidence, but is this because he doesn’t believe the message was real or because he knows it was directed at Chloe? Or is Chloe just becoming paranoid? She’s more concerned about the shrouded figure she thought she saw following her -- particularly when the students hear the gates of the cemetery creak back open behind them...

The next morning, the Doctor returns to Barry’s lab to review his experiments, and he and Leela bump into Finer on his way out. Finer has just informed Barry that thanks to his failure to produce results, and now Joan’s suicide, the Kellerfield foundation has cut Barry’s funding and the University has fired him. Leela follows Finer while the Doctor tries to speak with Barry, who is upset that the Doctor didn’t tell him about this personally -- and then irritated when he realises that the Doctor isn’t from the Kellerfield foundation at all, but a madman who claims that Barry’s experiments may bring about the end of the Universe. As he now has nothing better to do, Barry agrees to test the Clearspring Water samples, particularly when the Doctor points out that they’re bottled in Yorkshire, where Finer hails from. Meanwhile, the Doctor reviews Barry’s experiments, which confirm that the higher-than-average results of the initial tests seem to have been a temporary anomaly -- at least, until he sees the tape of the Zener card experiment. Barry, who doesn’t really believe in psychic powers, failed to notice that Josh was, for example, calling himself a star and waving at the camera moments before Tommy turned over a star and then a set of wavy lines. Not only did Josh correctly identify every card before it was turned over, he’s smugly hidden his abilities in plain sight...

Simpson plans to let Fred continue to cover up Joan’s death so he can catch him in the act and find out what the University authorities are afraid of. Bartok knows that Simpson has another reason to care about this case, for he’s learned that the students were trying to contact the spirit of Amanda Joslin, the girl found beaten to death in Norswood six years ago. Simpson never found the killer, at least not officially -- but he’s sure that Amanda’s birth father, a professor at the University, is responsible for the death. He’s vowed to bring him to justice, and he’d like to know how the students, newcomers to the area, knew about the murder. Bartok thus tracks down Ralph, Meg and Tommy in the student pub and buys them beer so they’ll tell him about the séance. They seem to recall that it was Josh who first suggested it, and deduce that he learned about the murder from Finer, his department head and Amanda’s father...

Leela has followed Finer to confirm her suspicions after glimpsing him at Barry’s lab, but even after getting a good look at his face she is afraid to confront him; why does he have the face of the demon who attacked him in the woods? While trying to work up her courage again, she meets Chloe, who is frozen in fear on a bench, convinced that that shrouded figure from the cemetery is stalking her. She accompanies Leela back to the parapsychology department, but the walk seems to take longer than it should, and Leela begins to hear strange rushing sounds and see the shrouded figure out of the corner of her eye. She follows the figure around a building only to find it suddenly looming in front of her; fortunately, her training kicks in at the last moment, she realises that its appearance doesn’t follow from its previous movements, and she narrowly stops herself from stabbing Chloe in the chest. She and Chloe then realise that the campus is deserted, and as Leela tries to focus on the strange rushing sounds, the demon’s voice screams abuse at her and then whispers promises to let her live if she leaves Chloe to die. Leela realises that this is all an illusion, and when she finds that she isn’t casting a reflection in the mirrored windows of a nearby building, she stabs the building with her knife and the illusion breaks. She and Chloe have walked out of the campus onto a nearby freeway; the rushing is the sound of tires on asphalt, and Leela narrowly pulls Chloe out of the path of two oncoming lorries. As Chloe recovers from the shock, she finally realises that the voice screaming abuse at her was Josh...

The Doctor now believes that Josh is playing sadistic mind games with his fellow students, but why? Barry returns with evidence that the independently wealthy Finer owns Clearwater Springs, and when he reveals that Finer ordered him to drop Josh from the programme, the Doctor becomes convinced that there is a connection. He thus drags Barry to the physics department, which is locked up for the holidays, but Barry thinks the Doctor’s mad and is thus looking the other way when the doors open up to let the Doctor in. Barry is too late to follow him, and he thus asks Bill Parnaby for help. Parnaby admits that Finer always acts strangely at this time of year, and agrees to help Barry convince security that the Doctor may be in trouble. They arrive at Fred’s office just as Chloe reports that Josh tried to kill her. Simpson has just caught Fred going through Joan’s room, under orders from his superiors to look for evidence of drug use and suppress it in the case of a public inquiry. Bartok points out that they don’t need a warrant if they suspect drug use, and Simpson leads the others to the physics department, to confront Finer and learn the truth at last.

The Doctor finds Josh waiting inside the physics department, and realises that he’s even more powerful than the Doctor had feared. The bulk of the department is located below ground in an abandoned bomb shelter, and Finer is waiting for the Doctor there, having realised that he’s someone who won’t be missed and thus perfect fodder for Finer’s master plan. Finer reveals that, amongst his other enterprises, he is the head of the Kellerfield Research Foundation; he used Barry to find a real psychic and disposed of him once he was of no further use. Since Barry was taking too long, Finer spiked the first free samples of bottled water with a drug known to cause mild psychosis as a side-effect; one of the psychoses was the illusion that the user had psychic powers, and Finer hoped that the delusion would kick-start the development of actual powers in the truly gifted. The Doctor is appalled by Finer’s lack of ethics, but he’s even more appalled when he sees that Finer has built what he believes to be a time machine but which is only the most dangerous half of one. Like the TARDIS, it collapses probabilities down to a single timeline, but unlike the TARDIS it has no control systems and doesn’t stop once it’s started. Local reality may already be beyond repair, and if Finer powers the machine up fully, the reaction will become self-sustaining and the Universe will be destroyed.

Finer refuses to listen to the Doctor’s attempts to warn him of the danger, and instead drugs him to keep him out of the way while he activates the machine. The Doctor awakens in a cellar full of rats, which Josh is manipulating to scare him; on Finer’s orders, Josh has still been taking the drug in order to ensure that his powers don’t fade, and as a result he has become dangerously psychotic. Finer has now activated the machine, and he still refuses to listen to the Doctor’s warnings, even when the Doctor correctly deduces that the machine is aligned with Norswood, where the original multiversal weakness was located. Finer finally admits that six years ago he beat his daughter to death in a fit of rage at her disobedience; torn with guilt, he needs to send someone back through Time to change the past, and he needs a psychic to keep tabs on the traveller. The Doctor is helpless to prevent Finer from dropping him into the time field, where he sees that he’s too late; the reaction is self-sustaining, and within seconds all possibilities in all timelines will cease to exist.

While Chloe reports her experience to security, Leela goes to the physics department to hunt down the Tesh who tried to kill her in such a cowardly manner. The door opens to let her in, and she avoids the enroaching voids and finds her way to a surveillance station which she tries to use to locate the Doctor. She then hears a pulsing sound and follows it to the experimental time machine. The machine is out of Finer’s control, but so is Josh, who plants the illusion of death in Finer’s mind, killing him before he can activate the fail-safe and destroy the time machine. Josh now believes that he has the power to destroy the Universe and thus make himself God, but as he prepares to trigger the final reaction, the Doctor arrives, having escaped from the time field thanks to the TARDIS. When he first arrived in Norswood, he set the TARDIS probability compensators to patch up the multiversal flaw he’d detected; thus, the TARDIS has materialised in the time machine’s pulse flow at the moment of crisis, and is providing it with the control systems which it lacked, thus saving the Universe.

The Doctor convinces Josh to treat him and Leela like the children they are in comparison to him, by giving him until the count of one hundred to escape. They thus get into the TARDIS moments before Josh gets bored and presses the button. Thanks to the TARDIS, the Universe is saved, but local reality has already been damaged beyond repair and this particular timeline is erased. The Doctor theorises that the temporal damage allowed Leela to get into the physics department by opening the door for herself from the surveillance room without realising it. It’s a moot point now; none of those events ever happened, and the Doctor believes that he and Leela will forget them when they next leave the TARDIS. In the timeline which now exists, John Finer turned himself into the police for his daughter’s murder instead of panicking and hiding the body in the woods; there was no Kellerfield Research Fellowship, Barry Hitchins became a media pundit after all, Joan Cox is still alive, and Josh Randall is an ordinary student with a mild interest in the paranormal. Perhaps one day he’ll get a grant to study it properly.