Three conventions are booked into the Pinehill Crest Hotel in Newmansgate, Kent. In the William Rufus Room, spiritual channeler Annie Carpenter is holding her first audience outside London, and her loyal follower Mary Hughes has invited her husband Brian to join her and see what Annie gets up to. Brian, the director of Hughes Avionics, is skeptical enough even before Annie gets to the chanting and the speaking with the squeaky voice of her spirit guide "Om". But as he mockingly shakes her hand, he suddenly begins to twitch -- and he too begins to speak with an alien voice. Annie ushers him into the back room to discuss his enlightenment, and once they're alone she lays into him, angrily demanding that he back off her pitch. But Brian isn't just putting on a fake voice to fleece money from the gullible; he is indeed channeling a spirit, one which Annie has encountered before, in her dreams. It's a gloating, evil voice which she must obey, and she has no choice but to watch as the spirit she has unwittingly summoned reveals its true form to her. The door she has opened in her mind grows wider, and soon she too has been possessed; the bridgehead and the demi-leader are here, and they will hide within their host bodies until their leader arrives. Mary arrives to check up on her husband, to find him and Annie both slightly dazed, as if waking from a trance. Both Brian and Annie have marks on their hands, like burns; a mystical stigmata like a pentagram. A shadow of something much larger than themselves, in both time and space...

The TARDIS materialises on the hotel roof, and the Doctor, Ace and Benny take the lift down to the foyer. It's 15 August 2000, and a policeman has arrived in response to a call which the Doctor sent a bit too early; the dead body he'd reported has not in fact arrived yet. He ushers Ace and Benny away from the lifts as one of them opens to reveal the body of what appears to be a homeless old man. As the policeman studies the body, the Doctor books rooms for himself and his companions under the name of Summerfield, and experiences a moment of dizziness due to a time experiment being conducted elsewhere in the hotel. In the Princess Diana suite, Michael Pembroke is demonstrating his temporal accelerator to a group of industrialists, while elsewhere Annie Carpenter is hosting an audience of New Agers and a cross-stitch convention has filled yet another set of suites to capacity. The Doctor sends Ace to Pembroke's demonstration and Benny to the seance, telling them that the three bookings are the things that any fire needs -- fuel, oxygen and a spark for combustion. And if they can't stop it, the human race will be consumed.

Ace slips into the Princess Diana suite, and watches quietly as Dr Pembroke introduces himself and demonstrates his temporal accelerator. At first it seems to be an ordinary particle cannon set up to fire into a vacuum chamber, but Pembroke proves that the proton fired from the cannon impacted with its target before it was fired from the source. The applications are obvious, and as long as Brian Hughes is there, the bidding to fund Pembroke's further work will be fiercely competitive. Brian himself asks a number of informed questions about the device, which works with theoretical particles beneath the quantum levels of reality -- by finding holes in the plot of the Universe, as it were. With this machine, instantaneous travel between realities may be possible. As the impressed audience breaks to look over the machine's technical specs and to share tea and biscuits, Ace tries to get a closer look at the machine, but Pembroke ushers her away when he realises that she's gatecrashing. As she goes, she sees Brian making purposefully for the machine, and privately warns Pembroke not to let him near its data settings. Pembroke ignores her, unsure where she came from and desperate to impress the very rich Hughes, but even so he begins to worry when Brian makes adjustments which even Pembroke doesn't understand. The machine begins to generate a timespace curve, and Brian locks off its controls so the programme can't be altered. He then departs, telling Michael that he has a few more errands to run, and the slightly unnerved scientist watches as the hum of power from the machine begins to rise...

Benny enters the William Rufus Room as Annie and her circle prepare to summon Om again. Thanks to Brian's interruption, Mary is skeptical of Benny's sincerity, but Benny assures them that she is there to contact her lost loved ones -- the two turtles who were once her only companions in isolation, and who died when the men she was hiding from came to arrest her. Annie welcomes her to the circle and tells her about Om, a high evolutionary from Rigel IV who exists partly in the higher dimensions; in this world, he takes the form of a golden-skinned man with blonde hair and silver eyes. The seance begins, but when Annie begins to speak with the voice of Om, Benny asks him if he knows Johnny Clarke, the guy who reads the weather on Rigel IV's nightly news. Annie tries to laugh this off, but it's as if the laughter itself is weakening her... or perhaps it's weakening the mark on her hand. When Benny tries to take Annie's hand, a very different alien voice speaks through Annie, telling her not to disturb the shadow, and Annie then retreats to the back room to recover. Mary is upset with Benny for disturbing Annie's concentration, and Benny apologises, realising that Mary genuinely believes in Annie's sincerity. Mary admits that she doesn't get out much; she and Brian don't have much in common. When she learned that Annie was booked into the same hotel as Brian's convention she'd invited him to join her in the hope that he'd start to share her interests, but he still doesn't believe, even though Om placed the mark of his power on Brian's hand as well. Benny is intrigued, and decides to have a private little chat with Annie.

The cross-stitch convention is booked beyond capacity. Gary Williams, the event's organiser, sends his chief steward Mike Duff to buy more attendee badges from the stationery store, and as Mike goes, the Doctor arrives to question Gary about the convention. He seems oddly irritated to learn that there are about 663 attendees... give or take a few. Meanwhile, Mike bumps into Ace in the lobby, and learns that nobody can leave the hotel until the policeman has finished his investigation. As the policeman sends the lift back up to the staff levels to ensure the security of the crime scene, Mike catches a glimpse of the old man's body and recognises him as old Will, a homeless man who used to live behind the hotel. Mike often bumped into Will while organising this convention, and remembers him as being so depressed that he could barely work up the energy to ask for change. And his luck wasn't improving; the last time Mike saw him there was a mark on his hand, like a burn or a knife slash -- a shadow in the shape of a pentagram...

Benny finds Annie "meditating" with the aid of a glass of sherry, and although Annie tries to cover at first she just doesn't have the energy to keep pretending, particularly when Benny does that thing with her eyebrows. She may be a fake spiritualist, but at least she gets people like Mary out of the house, and she's cheaper than a psychiatrist. Benny shocks Annie by asking out of the blue if she's pregnant, which she is -- and she fears that when she tells the father, he'll bolt. Benny is more concerned with the mark on Annie's hand, and the odd voice which Annie takes on when Benny asks about it. She promises to sit quietly if Annie lets her attend the next session to see what Annie can do, and Annie agrees, cynically assuming that Benny just wants to learn the tricks of the trade. The circle reforms, and Benny sits between Mary and Annie... but this time, as they all begin to chant in unison, something really does start to happen. Something is approaching, and as Benny tries to pry her hand free of Annie's, she sees that the rest of the circle is in a trance. Annie begins to transform physically as well as mentally, as the seance opens a conduit and summons the leader of the Scourge...

The channelers wake from their trance to find Annie transforming into a giant insect. Benny urges Mary to evacuate the others as quickly as she can, while she remains to find out what's happening... but she too flees when it becomes clear that the creature taking possession of Annie is not the peaceful being of light Om was supposed to be. It's grown very dark outside the hotel...

The Scourge leader and demi-leader order the staff and customers to leave the foyer, and they obey without question; it is physically impossible for the human nervous system to disobey the will of the Scourge. Ace, apparently realising that the Doctor has in fact sold out the Earth, storms off with Mike while the Doctor and the leader retire to the manager's office to go over the fine points of their contract. In the Princess Diana suite, Ace finds Pembroke trying and failing to shut down his machine; something is still working inside, maintaining a connection to wherever they now are. Benny arrives and tells Ace what happened at the seance, and Ace in turn explains that the Doctor's pretending to side with the aliens, and she's pretending to fall for it. Mike and Michael, completely out of their depth, accompany Ace and Benny back to the cross-stitch suite, where Gary's attempts to keep order fall apart completely when the demi-leader bursts in to spread fear. He has already killed the policeman, and as the attendees scatter in terror he traps Mike's friend Julie behind the displays. Despite the danger, Mike rushes to help, but the demi-leader tears Julie apart and orders Mike to put his fingers to his own neck. Mike can't disobey the voice of the Scourge, and Ace, knowing that nobody can save him, stops Gary from interfering as Mike impossibly strangles himself to death...

As the Doctor and the leader go over the fine points of their contract, the Doctor notices that the demi-leader is not present. He carefully doesn't react when the leader informs him that the demi-leader is off spreading fear amongst the humans, to create a suitable atmosphere for the next stage of the summoning. The bridgehead Scourge arrives in what used to be Annie's body, and is intrigued by the sight of the Doctor, whose body stretches back and forth across Time. It also questions the trivial reward he requested for the betrayal of Earth, a biodimensional implant which the Scourge use to torture humans by stretching them across possible timelines, but which will enable the Doctor's mind to roam the multiverse at will. Such a request is, however, characteristic of what they know of him. They've done a lot of research on the Doctor since they first encountered him dreaming on the astral plane. They have plotted to conquer this Universe and manage its resources more efficiently ever since they first became aware that its population was the Scourge's primary food source, and when they searched time and space for a suitable invasion point, they found the Doctor waiting with his offer. They are well aware of the fact that he often defends the human race by leading its enemies into traps, but he modestly points out that he's often much more intelligent than those he fights. Only a foolish species wouldn't see his traps coming, and the Scourge are, of course, nothing like that at all...

The Doctor sets off to collect the demi-leader for the focussing, and finds that he's arrived too late to prevent bloodshed. Now that he's here, however, Ace and Benny feel sure that the situation will soon be resolved. The Doctor sends them to his room to wait -- room 666, as it happens -- and assures them that the Scourge are so convinced of their superiority that they won't be bothering to look out for his traps. As they go, the Doctor calls out to the demi-leader, interrupting him before he can convert Mike's body into another Scourge soldier. The demi-leader remains suspicious of him, but accepts his claim that the leader needs him for the next stage. He has caused enough panic for the summoning of the Scourge to begin...

Ace, Benny, Gary and Michael wait for the Doctor to arrive, but Gary finds it hard to keep calm and have tea while monsters are herding his friends like cattle. Even Michael has the urge to go out and save the people he loves, but how can he fight things like the Scourge? Desperate to act, he turns on the television for the news, but there's nothing to be heard except the sound of cosmic background radiation, an echo of the Big Bang which should normally be drowned out by everything else in the Universe. Benny opens the window curtains to reveal that the darkness outside is absolute; they're not even in space, they're somewhere utterly empty, a complete, meaningless void. Perhaps, Michael theorises, they're in a higher mathematical dimension where the link between perception and reality is closer than in their own, which would explain why there is air and gravity here.

The Doctor arrives, just in time for tea, and finally explains what's going on. The Scourge are parasites from a higher dimensional plane, who prey on the depression and despair of lesser beings such as humanity; they are quite literally people's inner demons. And just as humans would be gods in a world of flat circles, the Scourge, if allowed to cross to Earth, would have the ability to walk around time, reach through walls and conquer the interior of human thought. Even in their current state they can communicate on a level which generates religious awe in the human nervous system, making it impossible to disobey their voices; should they achieve full materialisation and then return to Earth, humanity would have no choice but to worship them as gods of despair and horror. So the Doctor intends to stop them. He's already put a canister of pacification gas in the hotel's ventilation system, timed to sedate the occupants of the hotel at the very moment that the Scourge attempt to generate the crescendo of fear they require -- and he's also arranged for Michael's temporal engine to cut out at the same moment. Rather than materialising on Earth, the Scourge will be ripped apart across the dimensional rift, unable to reform themselves for millions of years.

Unfortunately, the Doctor has made a slight error of judgement. Instead of generating terror in the hotel's staff and the convention's attendees, the Scourge have summoned a single human to them -- Mary Hughes. The demi-leader blocks Mary's perceptions, making her believe that she's finally found her husband amongst the confusion. Annie tells her that Brian has joined them, and that they need Mary's help to open a focus and bring a wonderful new world into being. All they need is for her to concentrate on the space between their hands. Although puzzled, Mary does as they ask... and as her perception alters the space she is observing, the hard radiation generated by the quantum adjustments burns her from the inside out, showing her a vision of the Scourge's home as it does so. Mary dies in agony, seeing the fires of Hell before her, and the terror in her mind as she dies is enough to focus the Scourge and allow them to summon their army to them. On the edge of the reality interface, the army of the Scourge approaches, to swarm out of Hell and claim the Universe as their own.

Just as it appears that the Scourge have won, the Doctor falls into a coma; his breathing stops, his heartbeats slow to an almost imperceptible rate, and outside the windows, the darkness of the fractional dimension returns. Realising that they still have a chance to fight the Scourge, Ace orders Gary to strike her on the ears, bursting her eardrums so she will not be able to hear the Scourge's commands. The TARDIS medical bay can always put her right later. He reluctantly does as she asks, and it's one of the most painful things she's ever gone through -- and one of the bravest things Benny has ever seen. Benny, Michael and Gary hide the comatose Doctor in a storage closet, while Ace examines the canister of Pacificus and finds that there's a little bit left. Benny decides to find the bridgehead, the Scourge which used to be Annie Carpenter, and Michael is horrified to learn that Annie has become one of the monsters. As they set off in search of Annie, they see the demi-leader heading for the accomodation levels; the Scourge have realised that somehow the Doctor is keeping them trapped in this dimension, and the bridgehead is searching for his body while the demi-leader spreads further panic amongst the trapped humans in the hope of inducing materialisation.

The Doctor's allies find the bridgehead in the swimming pool, directly above the storage closet where they left the Doctor. Ace taunts the creature into attacking her, and the bridgehead, confident in its own powers, orders Ace to pluck her eyes out. Ace, who didn't hear a word it just said, attacks and restrains the shocked Scourge while Gary sprays it with the pacification gas. This calms Annie down enough to put the Scourge in her mind at bay, and Michael speaks soothingly to her, trying to keep her calm while Benny leads her to the closet where they left the Doctor. The Doctor said that mystics encounter the Scourge in their astral wanderings, and as it was Annie who provided them with their foothold in this reality, perhaps she's more of a psychic than even she realised. Indeed, when Annie places her hand on the Doctor's forehead, the Doctor is able to speak through her, telling Ace to get to the TARDIS and telling Benny to come inside and help him. Unfortunately, Annie loses contact before the Doctor can tell Ace what she's supposed to do when she reaches the TARDIS. Annie places her other hand on Benny's forehead, and the Doctor uses her as a conduit to pull Benny's mind inside his own. Ace and Michael set off for the TARDIS, leaving Annie in Gary's care.

As the demi-leader spreads further panic amongst the humans trapped in the hotel, the leader searches for the power which is keeping them trapped and locates a source of temporal energy on the hotel roof. He and the demi-leader disable the lifts and block the stairwells so that the leader may examine this threat without interruption. Ace and Michael find their way to the roof blocked at every turn, but, using a device issued in Spacefleet to pry the lids off Daleks, Ace pulls open the lift doors and she and the reluctant Michael climb up the inside of the shaft. Unfortunately, the demi-leader has been watching for such an attempt, and it clambers up the lift shaft after them. Ace and Michael manage to make it to the top and block the door behind them, but once out on the roof Michael is stunned by the sheer emptiness of the void around him. Ace ushers him towards the TARDIS, but as she reaches into her pocket for the key, the leader of the Scourge steps out from behind the police box, ready for them...

Benny arrives in the Doctor's mind, an echoing void full of tasteful furniture. The Doctor is waiting for her, needing her help to fight the Scourge which is trying to consume his thoughts. When he was attacked, he used a form of mental kung fu, in effect throwing the soldier into his mind, jumping in behind it and slamming the door. And since the Scourge are part of a group mind, if one is trapped then they all are. The Doctor is thus holding them in the fractional dimension and defining its nature, supplying those in the hotel with air and gravity. Benny notices images of people standing nearby, visions of the Doctor's past and future selves, and although she's quite taken with the image the Doctor identifies as his eighth self, he warns her that that's only a possibility. As things stand now, it's much more likely that he will be devoured by the Scourge, and that the Earth will be destroyed.

As they head for the battle, Benny questions the Doctor about Annie, afraid that he's going to use her as a pawn to defeat the Scourge, harming her and her baby in the process. He assures her that this is not the case, if assures is the word; in effect he's admitting that his only plan has fallen apart completely, and that by underestimating the Scourge and overestimating himself he may have caused the destruction of Earth. But as Benny points this out she realises almost too late that the Scourge within the Doctor's mind is feeding on his own anguish and despair, and that she's only adding to it. She tries to keep the Doctor's spirits up as they approach the Scourge itself, a gigantic alien insect cutting deep into the Doctor's brain and undermining his sense of self-worth. As Benny tries to get a closer look at the enemy, she stumbles and knocks one of the Doctor's memories over the edge of his mind into the pit. It was a simple thing, the smell of a flower in a small English village, one of the little memories that make up a life... now gone forever. And as it goes, the Scourge opens its eyes, sensing the Doctor's despair. The Doctor struggles to deny its power over him, but it's too strong for him, and even with Benny's help the Doctor is unable to deny the truth of his own failures...

Gary puts Benny's physical body in a staff bedroom, locks it and slips the key-card under the door, hoping that this will be enough to keep her safe. But he knows it isn't, and lashes out at Annie, frustrated, before remembering that he's supposed to be keeping her calm. Every time she moves she's reminded of how she's changed, and the Scourge in her mind is whispering to her, telling her that she deserves this for what she's done. It's also telling her about Gary, who fiddled the accounts of the convention and stole two thousand pounds from people who thought he as their friend. He can't deny this, and he's run out of pacification gas -- and the Scourge within Annie seizes control of her once again through her despair. Triumphant, it orders Gary to put his hands to his own neck and throttle himself to death; it should be impossible for a human to do this, but a Scourge commands it, and deep down Gary believes he deserves to die for what he has done. The bridgehead departs, leaving Gary to die and leaving the Doctor unharmed as well; for soon, the Time Lord will be one of them as well.

Michael and Ace are trapped between the triumphant leader and demi-leader, who have just felt the Doctor and the bridgehead join their consciousness once again. The leader calls Ace to him and orders her to turn her knife against herself -- but she can't hear him, and to his shock she uses the knife to stab him instead. He staggers away, wounded, and orders Michael to grab Ace and throw her over the edge of the roof. It appears that Michael has no choice but to obey, and deep down he too believes that he deserves what's happening to him, because he cheated on his wife. Ace, unable to break free of him but close enough to read his lips as he admits the truth to her, tries to convince him that he doesn't deserve to die. People don't always deserve what happens to them, and just because he's done something wrong that doesn't mean he has to listen to the Scourge telling him that he's worthless. Nevertheless, it appears that Michael is going to throw her over the edge of the building...

But just as they reach the roof, Ace hears the Doctor's voice, speaking directly into her mind through Michael's. The Doctor fell into the darkness to get closer to the Scourge, trusting that there would be something to catch him at the end of his fall. Now, he is a part of the Scourge group mind, and when the Scourge tell somebody to obey, he can make sure that it's him they listen to. He tells Ace to pretend to struggle, and before the Scourge realise what's happening, Ace and Michael have "fought" their way across the roof to the TARDIS -- and inside. There, as Michael tries to comprehend the interior dimensions, Ace finds that her eardrums have been healed; the Doctor suspected that they wouldn't escape the Scourge unscathed, and had the TARDIS medical bay focus its nanites on the control room. Using Michael's fingers to point at the relevant controls, the Doctor directs Ace to rip out certain safety catches. The Scourge are inside the building, the building is inside the fractional dimension, and the dimension is inside the Scourge; and by altering the TARDIS safety protocols they will be able to mix the dimensions together, making it all bigger on the inside than the outside. Then, they will have a chance to save the world.

Benny awakens in her own body, and rushes back to the storage closet, where she knocks Gary's hands from his throat, saving his life. But the Doctor has been left unharmed, which means the Scourge must know he's one of them now. She and Gary set off after the bridgehead, and find it terrorising more people in the accomodation levels -- but as they watch, helpless, the TARDIS materialises, warping space around it as it does so and knocking the bridgehead unconscious. Benny and Gary help Ace to carry the bridgehead into the TARDIS, where the ship's temporal grace circuits and the medical nanites free Annie's mind from the Scourge once more. Unfortunately, now the other Scourge will have their defenses up, and they can't leave this dimension in any case while they and the Scourge are all twisted up around each other. To Benny's horror, she feels a knot of despair within her own mind, as do the others; it's the worst thing they've ever felt, and the Doctor has done this. But he's done so for a reason; now they and the Scourge are truly equal, and the Scourge cannot force them to listen any longer. The Doctor tells them to get everybody to the foyer; he still has to fight his own Scourge, but now they have the power to save themselves. He releases control of Michael's body, but as Ace and the others head to the foyer, Benny takes Annie back to the closet, telling Annie to put her back in the Doctor's mind. She won't leave him behind, whatever the risk. Annie wishes anyone cared that much about her, and before leaving, Benny advises Annie to tell the father of her child the truth; then she'll know for sure.

On the roof, the leader and demi-leader recover from the shock of the TARDIS' departure to find that the nature of this dimension has changed around them. They can no longer reach through space or time; it is as if they have lost their senses. But they are still physically stronger than the humans, and once they've proven their superiority they will change the nature of the cosmos and their power will be restored. In the foyer, Annie, Ace, Gary and Michael hold the line as best they can, but it's difficult to be leaders with the Scourge whispering in their heads, reminding them of their failures. Gary stole from his friends, Michael lied to his wife and kept her trapped in a loveless marriage, Annie invented Om to make people pay attention to her. But Annie, at least, knows that this isn't the whole truth. She may have lied, but she really is psychic, and has a power she wasn't aware of before. This is all they have to hold onto, as the Scourge approach...

In the Doctor's mind, he is struggling with his own Scourge, which tells him that he's merely a troublemaking child who fears being alone so much that he deliberately alienates his companions to see whether they'll leave him or not. But Benny arrives to tell him that what the Scourge is telling him is not true, unless he believes it. Fear can stop you doing anything, but love can stop the fear, and the Doctor will always have his friends. The Scourge, sensing that it is losing the Doctor, lashes out at Benny, but the Doctor grabs her mind and pulls her back; she wouldn't leave him, and he won't leave her. The Scourge feed on despair, and to defeat it they need to have hope, trust their friends, and believe that their sins will be forgiven. It's the small details of life that make it meaningful; not the things we do or leave behind us, but the lives we live while we're here. Annie is going to have a baby, and that hope for the future is what gives Benny the strength to pull free. She and the Doctor leave the Scourge wallowing in the darkness, and return to their lives, to save the world.

The leader and demi-leader arrive in the foyer, and are merely amused when the humans try to stand up to them. The Scourge have fed on humanity for millennia, and know how weak they are. But before the Scourge can reveal the truth, Gary does so himself, confessing to his friends that he's stolen from them. Michael has cheated on his wife and Annie has lied about her power to true believers, but they're only human, and they're not going to let their weaknesses devour them. As the assembled guests face up to the Scourge, refusing to give in, the demi-leader finds the shadow on his hand fading away, and Ace takes advantage of their confusion to attack them. But the leader strikes back, and captures her. It is about to tear her apart when the Doctor arrives... and although they have his friend captive and are threatening to dissect her before his eyes, he isn't in the least bit afraid. He's gone through the worst they can do to him, and he's seen the truth; his friends give him the authority he needs to play with the fate of worlds, because he's the Doctor, and he always wins. He reveals to Michael that Annie is pregnant, and once Michael gets over the shock he's delighted... and he's damned if he's going to die, now that he's learned he's going to be a father.

Now the Scourge are seeing the other side of humanity, and their army is losing its grip on this dimension. Only the leader and the demi-leader remain, and the Doctor promises to show them mercy if they abandon their invasion now. The leader mocks his arrogance, and prepares to tear Annie and her unborn child apart -- but the Doctor stops it in its tracks with a word. The Scourge no longer have power over people unless they listen to them, but the Doctor can tell the Scourge just what to do. He reminds Brian of who he really is; many innocent people are dead, including Mary, but Brian does not bear the blame. The Doctor's voice puts Brian at peace, and the demi-leader is banished from his body, never to return. Now only the leader remains, and the Doctor again offers it mercy... and when it refuses to give in, the Doctor simply tells it, quietly, to let go. The leader cannot hold on, and it dissipates across the dimensional rift, to be scattered for millennia. All that remains, all that the people were ever afraid of, is a homeless old man who died alone in the dark. The Scourge have gone from their heads, Annie, Brian and the Doctor have returned to normal, and light breaks outside the hotel as they return to the Earth, safe at last.