Thomas Brewster’s earliest memory is of his mother’s funeral. He thinks he must have been about four or five, but it’s hard to judge when you haven’t had any birthdays. He can remember hiding in the parlour and trying to ignore the heartless comments of his aunts and uncles as if it was yesterday. They blamed him for the fact that his mother killed herself and they all made excuses for not being able to take him into their families. His mother was laid out in a box, dressed in black and with a veil over her face, but his uncle decided it was time Thomas faced up to the harsh realities of life and lifted the veil to show the young boy what five days in the water can do to someone. Her face was whiter than anything Thomas had ever seen and her lips were pale blue. There were rough holes in her skin where the fish and the eels had taken a bite. Thomas started to cry and his family were satisfied.

Unnoticed in the crowd were the Doctor and Nyssa. They spotted Thomas, but when they discovered the year was 1851, they realised they’d arrived too early.

The memory of his mother’s face stayed with him forever and every time he tried to remember what she looked like, he saw her as she was then, bloated and swollen. Even as his mother’s body was buried in the ground, the family continued to bicker and blame her for being selfish. Thomas has one other memory from the day. As he was walking away from the grave, he saw a strange blue hut in the distance under the trees. It had a lamp on top which started flashing, and then the hut wasn’t there any more.

The family agreed amongst themselves that Thomas should be sent to the workhouse, to be cared for by the parish. Being such a miserable creature, they felt it was no more than he deserved. The boy soon found himself dossing in the ’Spike’ with twenty other orphans and paupers. Years passed and he became used to the routine of prayers, working in the laundry, and receiving lessons from Mr Shanks, the master of the house. Thomas was terrorised by the man and was regularly beaten for not paying proper attention.

The days were hard, but the nights were worse - shivering for warmth under a woollen blanket and listening to the inmates below, sharing their night terrors with anyone who cared to listen. Then one icy, black night, after he’d been there five years or so, Thomas heard a woman’s voice singing “Oranges and Lemons”. The voice called out Thomas’s name and pleaded for him to save her, then she began screaming. Although he couldn’t remember his mother’s voice, he was certain it was her. The only thing he couldn’t be sure about was whether it was a dream.

Then one cold Winter’s day Thomas received a brutal punishment from Mr Shanks when his attention in prayers was distracted by something out of the window. What had caught his eye was the tall blue hut that stood in the snow-covered courtyard down below. It was the same hut he’d seen years earlier at the graveyard. Later that day, Shanks had two unexpected visitors…

The Doctor and Nyssa asked to speak to Thomas Brewster, but Shanks said this wouldn’t be possible as the boy had been entrusted to his custody. Nyssa assured him they had no interest in anything Thomas might say about what happens inside the workhouse and the Doctor said he simply wanted to know if the boy had mentioned anything about having bad dreams. They started to ask the man about supernatural nocturnal visitations, ghosts, voices and hallucinations, but Shanks believed they we wasting his time and asked his assistant, Mr Sloop, to escort them from the premises.

Thomas had overheard the conversation between the visitors and Mr Shanks, but none of it made much sense to him at the time. Another year passed, then another, and every few months he’d have the same dream again. Each time he’d sit up in bed, alone in the darkness, and hear his mother’s voice singing “Oranges and Lemons” and each time her voice would sound a little bit nearer and she’d get a little bit further into the song. And then one night, he found he wasn’t alone. There, at the foot of his bed, someone was standing, watching over him. At first he couldn’t make out much more than just a shape, but then he could see the dress, the bonnet and the veil that covered his mother’s face. She called his name and pleaded with him to save her, then she screamed again…

Brewster has been at the workhouse for ten years when he is summoned to Mr Shanks’ office and told it’s time he went forward into the world. Shanks introduces him to a man called Creek who has ’generously’ offered the boy an apprenticeship as a river-man. The strange man looks over the boy and although Shanks assures him Thomas is a most healthy specimen, Creek complains that he’s paying for nothing more than skin and bones and thinks Shanks should be paying him to take the boy off his hands. He’s had experience of taking on workhouse boys before and says they’re more trouble than they’re worth, but Shanks persuades him that Thomas is different. Creek eventually agrees to the arrangement, although he still believes he’s doing the workhouse a favour rather than the other way round.

Creek takes Thomas to Jacob's Island, a notorious rookery on the south bank of the River Thames. They cross over a gangplank to Creek’s shop and the man warns Thomas to watch his footing as the mud around here is like quicksand and if he falls in, he‘ll never get out again. The plank is suspended over a ditch of slime and mud, on which weeds and rotting lumps of meat are floating. Creek tells Thomas his lodgings are below and says he’ll be joining a gang of other boys, but he carefully avoids explaining what the boy’s new job actually entails.

Thomas goes downstairs and is welcomed as a new recruit by the others. One of the boys, Pickens, shows him to a bed, which he says is still warm from the last lodger. He tells Thomas not to worry too much about the rats and says if they get too close, he should poke them with a candle. When Thomas asks what their job is, the boys all laugh. Pickens explains that they’re ’mudlarks’ and their job is to scavenge anything that’s been washed up on the banks of the Thames during low tide after being thrown off the back of trade ships to avoid the revenue men. They can expect to find all sorts of things from barrels of brandy to silk, lace and tobacco. Thomas wonders if this is stealing, but Pickens says the stuff doesn’t actually belong to anyone, or at least no one would admit to it. But to be on the safe side, they only work at nights and Pickens says that if anyone asks Thomas what he’s doing, he’s to say he’s a river-man. Thomas asks what Creek is like and Pickens says he treats them fine if they stay on his right side, but if anyone answers him back he’ll slate them till they don’t know their own name. That’s what happened to the last kid and that’s why there’s a vacancy for Thomas to fill. A bell rings signifying midnight and Pickens tells Thomas it’s time for them to start work.

Thomas and the other boys clamber down the oily steps to the Thames, each of them clutching a lamp to help them find their way through the thick, swirling fog. Pickens tells Thomas to keep his lamp low so it can’t be seen by anyone and warns him to watch out for dead bodies as all the people who jump or fall off the bridges tend to get washed up round here. Thomas suddenly goes into shock when he sees the shape of a woman, dressed in black, standing a dozen or so yards up the riverbank. He hears his mother’s voice singing “Oranges and Lemons” then she calls for his help and screams again. The woman jumps from the bridge and disappears into the water. Thomas drops his lamp, which smashes, and Pickens tells him he’ll be punished for sure when they get back. Thomas asks Pickens if he saw the woman watching them, but just then they hear a whistle from one of the other boys, which means they’ve found something. They join the boys and Thomas is surprised to see it’s the same blue box that he’s seen before, now half-buried in the silt and the slime.

Back at the shop, Creek is impressed by what his boys have managed to salvage. It took them half the night to drag the heavy blue box back, which makes Creek think it must be full of valuables. Pickens has already tried opening it, but he says the lock isn’t normal and it’s like nothing he’s ever seen before. Creek is dismissive of Pickens’ claim to be a master lock-picker, but the boy insists that if he can’t open it, no one can. Creek suggests prising the door open with a wrench, but Pickens has tried that too. They examine the mud on the box and realise it must have been in the river for years, but as they clean it up they reveal the words ’POLICE BOX’. Creek is more determined than ever to find out what’s inside, but first he wants to know how his new boy got on. Pickens makes an excuse for Thomas and says he had a funny turn and lost his lamp. Creek is furious and begins beating Thomas with his stick…

Creek sets Thomas to work scraping clean the goods that come in, and every day he finds another reason to beat him with his stick. This is Thomas’s life for months, stuck in his shop and never seeing daylight. Then one night, it all changes. The shop door opens and the Doctor and Nyssa enter, calling for service. Creek thinks the Doctor must be a revenue man and argues that he’s just a humble river-man, but they tell him they’ve come to see Thomas Brewster. Creek says he doesn’t know anyone of that name, but the Doctor says it’s vitally important they get to him tonight. Creek pretends to rack his memory and then tells them he used to have a boy of that name, but says he left some time ago after stealing from him. The Doctor says he’s prepared to compensate Creek for his loss if he ever sees Thomas again. Creek assumes Thomas has got himself in some sort of trouble, but the Doctor says he’ll only be in trouble if they don’t find him.

Creek makes an excuse to show the Doctor and Nyssa out, then he grabs Thomas and demands to know what he’s been doing. He assumes the boy has either been stealing or has run away from another master, but he knows they’re prepared to pay for information and he wonders what Thomas might be worth on the open market. He decides to sell the boy and tries to drag him out of the shop, but Thomas resists and they end up struggling. Creek drags Thomas to the riverside and tells him the previous boy died after falling into the mud and drowning. They continue to fight and Creek swings at Thomas with his stick, but then he loses his footing and falls into the river. Immediately he’s gripped by the mud and despite struggling, he’s sucked further and further into the filth. Thomas holds out the stick for Creek to pull himself free, but the man can’t reach it and eventually he disappears from sight under the mud.

Thomas returns to the shop and tells Pickens that he’s leaving and that Creek has had an accident and won’t be around anymore. Pickens asks if he can go with him, so together the two boys run to the wharf where Creek’s boat is moored and they head off for Limehouse, Wapping, or anywhere else but here. As they row for their lives, three figures appear on the jetty and call for them to stop. The Doctor and Nyssa warn Thomas that he doesn’t realise the danger he’s in, but in just a few moments the three of them disappear from sight, shielded by the dense fog. Thomas has never seen a mist like this before - a heavy, rolling pea soup, yet black as smoke and twisting as if it were alive. It quickly swallows up the boat and the boys start to panic as they can’t see where they’re heading.

The Doctor leads Nyssa down yet another dark corridor in search of the TARDIS’s auxiliary power station. She remains convinced that they’re lost, even though he says he knows the ship like the back of his hand. The TARDIS is suffering from a power cut, so they’re forced to use a torch to find their way. The Doctor isn’t even sure what the cause is as it could be any number of things, but first the Doctor wants to rule out the possibility of an internal fault. They try another door, but it only leads to the wine cellar. Then they try another and this time they find themselves inside the auxiliary power station. The Doctor looks around and discovers the Vortex shield relay has fused and something is leaking. As he examines the equipment, Nyssa is distracted by the sound a woman singing what appears to be a nursery rhyme. Unfortunately the Doctor says he can’t hear anything and until he’s repaired the problem, the ship can’t materialise, so investigating this problem has to be his priority. Then the woman’s voice becomes louder and this time the Doctor hears it too. Suddenly there’s a massive time breach and the Doctor says something has broken into the TARDIS. Nyssa can see a woman in black walking towards her. The Doctor urges Nyssa to avoid any physical contact, but the woman reaches out her hand and touches Nyssa…

…then suddenly Nyssa finds herself alone in the middle of a bustling London street. A market trader named Hartwright helps her to her feet and, assuming she got lost in the fog, tells her she’s in Seven Dials and says the date is November 14th 1867. Hartwright warns her this is no place for a lady. Not far away, Thomas and Pickens have spotted Nyssa and argue over whether she’d make a good target for pick-pocketing. Thomas insists they concentrate only on what they came for. They approach a pawnbroker’s shop and Thomas points to a barometer in the window. They smash the glass, grab the apparatus and flee, closely followed by a furious Hartwright. The boys dodge round him and knock Nyssa to the floor, forcing the trader to stop and help her back to her feet while they get away. He realises it’s not worth chasing after them as it’s easy for them to hide in the fog. A young man with a Scottish accent approaches Nyssa and says he’s been sent to collect her. He introduces himself as Robert McIntosh and says her arrival has been anticipated by a mutual friend who’s most eager to renew their acquaintance. He offers to escort her and they both leave.

As soon as they know they’re no longer being chased, Thomas and Pickens stop to admire the equipment they’ve stolen. Pickens thinks they could earn a few pennies from it, but Thomas says it’s not for sale. He won’t explain why they stole it, but says they have a few more items to get before they’re done.

At the Royal Society, the Doctor is coming to the end of a speech in which he unreservedly commends a paper written by Leon Foucault on the subject of calculating the speed of light. The audience gives him a round of applause and as he leaves the hall, the Doctor is joined by Robert McIntosh, who he admonishes for missing a really good lecture. Then the Doctor sees Nyssa and is delighted to discover she’s alright. She admits to being a little confused so the Doctor sends Robert to collect his hat and coat from the lecture hall while he explains what’s being going on. The Doctor tells her he’s been going under the name of Dr Walters, a small but necessary deception to enable him to make his way in this civilisation. He‘s even grown a beard since they last met as he found people weren‘t taking him seriously because he was “too young“. He thinks it makes him look distinguished and Nyssa agrees that she likes it. Robert returns with the Doctor’s hat and coat and the Doctor reveals to Nyssa that Robert is his new assistant.

Thomas and Pickens arrive at their destination, 107 Baker Street. Unfortunately the owner is just arriving home, so they’ll have to bide their time. From a convenient hiding place, they watch as the Doctor enters the house, accompanied by his companions. Nyssa is surprised that the Doctor has a house and he promises to explain everything after a cup of tea or maybe something stronger. The two boys are amazed to see Nyssa again after their encounter at Seven Dials and Pickens is immediately suspicious of Thomas who claims not to know she’d be here. Thomas gets a strange feeling about Nyssa, as if he’s seen her somewhere before, and Pickens teases him and says he may have seen her in his dreams.

The Doctor asks Robert if he minds leaving him to discuss matters of some delicacy with Nyssa. Once they’re alone, the Doctor explains that Robert is his assistant-cum-protégé and says it would be unusual for a pillar of the scientific community not to have one. Nyssa finds the whole situation odd, especially as she only left the TARDIS two or three hours ago. The Doctor tells her that after the time breach, he managed to restore enough power for an emergency materialisation, but the TARDIS brought him here to London twelve months ago and he’s been here ever since, living the life of an English gentleman. In order to gain access to the materials he needed to repair the ship, he was forced to become a member of the Royal Society. He takes her to another room where the TARDIS is standing in the corner, safe and sound. Nyssa wonders whether the Doctor’s “assistant” finds it strange that he keeps a police box in his study, but the Doctor says he’s never brought the subject up and is probably just being discreet.

Pickens asks Thomas what it is they’ve come to steal this time, but Thomas says he doesn’t know yet. Just then, the voice of his mother speaks to him inside his head and explains that the item she wants him to recover is an electromagnetic field generator. Pickens notices Thomas’s distraction and asks him if he’s seen a ghost…

The Doctor shows Nyssa his electromagnetic field generator and says it’s the only one like it in the whole of England. He says he’s borrowed it unofficially from a friend, James Maxwell, which effectively means he’s stolen it. The Doctor tells her that if you make certain adjustments, it becomes a surprisingly good field interface stabiliser which he needs because the one in the TARDIS has seen better days. Nyssa rebukes him for using equipment ahead of the scope of this civilisation, but he promises he’s been very careful to only tell people things they already know. During the year that he’s spent waiting for her, he’s been involved in other pursuits and shows her a device he’s particularly proud of. Using only contemporary technology, it detects disturbances in the space-time continuum. Nyssa identifies it as a Vortex Interferometer and realises this is how he knew where and when she’d arrive. Robert joins them with some tea and asks Nyssa if ‘Dr Walters’ has been telling her about their little mystery? He shows her a newspaper report about a house-breaker in Bayswater who’s just struck again. There have been a serious of inexplicable thefts of scientific paraphernalia over the past two years that have left Scotland Yard baffled. Nyssa assumes it has something to do with the Doctor, but he assures her he’s totally innocent. The missing items include metallic minerals, achromatic lenses, parabolic mirrors, electric batteries and even telegraph wires. The peculiar thing is that the thief has often left behind more valuable items, as if they were stealing to order, but no one knows what they intend to do with it all…

Robert produces a complete inventory of all the stolen goods. Nyssa can’t see any obvious connection between them, but she’s very tired and the Doctor suggests they talk again tomorrow. Robert offers her his room but Nyssa says she’d prefer to sleep on the couch in the Doctor’s study, so Robert bids them goodnight and retires to his room. Once they’re alone, the Doctor hands Nyssa the TARDIS key and tells her he’ll be in the kitchen if she needs anything. Moments after he leaves, the window opens and Thomas and Pickens climb in and grab hold of Nyssa, warning her not to scream. While Pickens holds a knife to her throat, Thomas searches around for the electromagnetic field generator. They’re about to leave when Thomas recognises the blue box in the corner of the room. Pickens suggests they leave straight away, but the voice of Thomas’s mother tells Thomas to take the girl too as she may be useful.

They head for the front door, but it’s locked. Pickens hands Nyssa over to Thomas while he picks it, but this gives Nyssa the opportunity to cry out for help. They open the door and race out into the street just in time as the Doctor and Robert both respond to Nyssa’s call, Robert checks outside, but there’s no one to be seen. The Doctor soon discovers the electromagnetic field generator has been taken too but he’s sure the thieves can’t have got far so they go out into the street. Unfortunately the fog is so thick they can’t see far in front of them. They try to work out where someone would go if they wanted to make a quick getaway and Robert suggests they’d go underground.

Thomas and Pickens take their hostage down into the London Underground and wait until the last train of the night on the Metropolitan Line pulls into the station. They order Nyssa onto the carriage and then the locomotive starts to move away. The Doctor and Robert arrive just in time to see it depart, but they both race down the platform and jump onto the moving train before it disappears into the tunnel. Once they’re aboard, they start to move down the carriages in search of their prey.

Nyssa starts to choke from the steam and Thomas tells her it’s one of the delights of underground travel. Pickens spots the Doctor and Robert searching the compartments and realise they’ve been followed. They open the carriage door and leap from the moving train, forcing Nyssa to jump with them. The three of them force their bodies flat against the tunnel wall and watch as the train passes by and disappears into the darkness. Thomas thinks they can’t be far from Portland Road so he lights a match and they start walking. He explains that when the builders dug out this section of the railway, they used it for storage and after it was abandoned, he and Pickens moved in. They arrive at an area adjacent to the tunnel and the boys tell Nyssa this is their home.

The Doctor and Robert have finished searching the train and realise Nyssa isn’t here. They wonder whether they made a mistake and perhaps the thieves doubled back, but the Doctor is certain they came this way. At first he suggests they get off at the next stop and head back for Baker Street, but then he has another idea and tells Robert to follow him.

Nyssa asks the boys how they’re able to survive down here without light or ventilation, but Pickens tells her this is a palace compared to Creek’s place on Jacob’s Island. He says it was Thomas‘s idea to chose this as a hideout and in fact he was most emphatic about it. But Thomas isn’t listening to them. The voice of his mother inside his head orders him to complete construction of the device, so he leaves for another room at the back where he keeps all the equipment they’ve stolen. He regards it as a private place and doesn’t like anyone else going in there. Nyssa noticed that Thomas had gone into a trance for a moment and Pickens agrees that he gets these ’turns’ from time to time. They used to be only occasional, but recently it’s been happening every day. He thinks they’ve done quite well for themselves and they’ve watched out for each other, but he admits to Nyssa that he likes Thomas more than Thomas likes him. Nyssa thinks she understands what he’s saying.

In the back room, Thomas is following the instructions given to him by his mother’s voice. He attaches the electromagnetic field generator to the rest of the equipment and she tells him it’ll act as a field interface stabiliser. His mother congratulates him and tells him the work is finally complete. This means she can now come back to him. She tells him to engage the power and he switches on the device…

Nyssa and Pickens can hear the throbbing of the equipment and they race through into the back room only to stop in their tracks when they see Thomas standing before the activated machinery. Lights flare around the room and electric sparks arc out from the equipment. Nyssa tells Thomas he has to stop what he’s doing right now, but he refuses to listen. She orders him to disengage the power as the mirrors and lenses are focusing the static charge and he’s tampering with forces he doesn’t understand.

As they return to Baker Street, Robert tells the Doctor they ought to call the police - after all, there has been a break in and possibly a kidnapping. Just then they hear a strange noise coming from the Vortex Interferometer. The Doctor explains that it detects disturbances in the space-time continuum, which means someone has just punched a hole in the Universe! He remembers the time breach and realises someone here in London is operating a time machine…

Thomas calls out to his mother and pleads with her to come back. For the first time he notices Pickens lying on the ground and Nyssa tells him his friend is dead, probably from suffocation. Thomas says none of this was supposed to happen, but she tells him Pickens died trying to save his life and they have to get away from those evil creatures.

The Doctor tells Robert that his machine was designed to create an access point for a corridor linking one time period to another. Robert laughs and says the idea is absurd, but fantastic. He wonders whether the corridor leads to the past or the future and the Doctor admits that he’s not sure. He looks again at the list of the stolen items and realises they’ve all been used in the construction of a time machine, but no one from this century could possibly have the expertise. Robert deduces that it was constructed by someone from another era, but their priority should be Nyssa, who is still missing. The Doctor believes that with the Interferometer they can pinpoint the location of the time breach which will tell them where she’s been taken.

Nyssa and Thomas continue running until they’re safely clear of the smoke creatures, which Nyssa guesses are a lifeform based on suspended gas particles. She’s surprised that he wasn’t expecting all this to happen, but Thomas says his mother told him that if he did what she instructed then she’d come back to him. He explains that she committed suicide by jumping off Southwark Bridge when he was four or five years old. Nyssa says she’s sorry, but she’s also confused because Thomas talks as though he still speaks to his mother. Thomas admits that he’s seen her a few times after she died and he hears her voice as if in a dream, but he insists that she’s not a ghost and that she’s trapped, as if caught somewhere between being alive and dead. He‘s been trying to bring her back into this world, but he had no idea what the machine he built would do.

Just then, the angry voice of a guard calls out to them from the nearby Underground platform and orders them to get out of the tunnel. Thomas warns the guard away and Nyssa tells him there’s been a toxic gas leak and advises him to leave for his own safety, but he refuses to move. Suddenly the gas creatures emerge onto the platform and surround the guard. He calls for help, then struggles for breath and collapses to the floor, dead. Thomas helps Nyssa up onto the platform just as the Doctor and Robert arrive. The Doctor identifies the creatures as a form of sentient miasma and Robert says it’s like the fog itself is alive. Thomas tells them it’s been killing people and the Doctor asks who the young man is. Nyssa says they don’t have time for introductions, so they leave the station as quickly as they can.

Outside the station, they pause to breathe in the clear night air. The Doctor asks for help closing the gates even though he knows it’s unlikely to stop the gas creatures for long. The Doctor realises Thomas is the thief who broke into his house and stole his electromagnetic field generator, but Nyssa says this isn’t important any more and in any case the equipment didn’t belong to the Doctor either. Robert is forced to agree that they have other more pressing concerns now, so they agree to go back to Baker Street where Thomas can explain everything!

Thomas tells the others that his mother claimed to be in the year 2008 and Robert assumes this must be where the ‘death wraiths’ originated. He wonders what kind of world could have such beings in it. The Doctor reveals that he’s visited Earth in that year and knows there were no creatures like that there, but Nyssa reminds him that he’s always said the future is indeterminate and is subject to change. The Doctor concludes that they’re dealing with an ontological time loop. In a quantum Universe all futures are possible, though some are more probable than others. Although it’s a very remote possibility, one of those futures must be one in which these creatures control the Earth, and if they have the capacity to send information back through time - for example, instructions on how to construct a time corridor - then they can use that time corridor to travel back into the past to influence events and make their version of the future more likely. The greater their presence in this time, the more likely that what was once just a possibility will actually become a probability, then a certainty. By invading the past, they’re creating the future from which they will invade the past. Robert finds the theory baffling, but fascinating, but wonders what this has to do with Thomas seeing his mother. The Doctor theorises that it’s a psychic projection from the future. Given the very low likelihood of their future, the smoke creatures would require an astronomical amount of energy to communicate through time. They’d need to find someone who was particularly receptive and was willing to follow their directions. So they made sure they appeared to Thomas as the one person he most wanted to see - his mother. Thomas insists that it really was his mother he saw and not some sick fantasy in his head, then he storms out of the room in anger.

In Seven Dials, the market trader Hartwright encounters an old friend, Judy, as she waits on the street corner. He advises her to get back indoors as she’s unlikely to see much business tonight with this fog, but she tells him she’s arranged to meet a regular. Suddenly a horse-drawn carriage appears out of the mist and crashes into a nearby wall. The horses are killed outright and Hartwright realises they must have been leading the carriage blind without a driver. They wonder what could have caused the horses to bolt like that, but then the mist itself seems to come alive. Despite there being no breeze, the fog swirls towards them and a glow appears from within. Hartwright and Judy realise this is no normal London fog and begin to panic, but as they bang on the nearest door and cry out for help, the fog overpowers them. They scream in agony as they’re completely enveloped…

Nyssa joins Thomas and assures him they didn’t mean to upset him, but he doesn’t believe her and says she can’t know what it’s like to have no mother or father and to be alone in the world. She tells him her parents were taken from her too, but she can’t bring them back even if she wanted to and they wouldn’t want her to waste her life grieving. She plans to make a future for herself, not to forget them, but to honour their memory. Thomas says he never even saw his parents, except for when he saw his mother’s body laid out. Nyssa hugs him and tells him that whoever’s been speaking to him, it’s not his mother but something powerful and evil using her form. Just then, Thomas hears his mother’s voice in his head again, telling him not to listen to the girl. Nyssa stops comforting Thomas when she spots the smoke creatures on the rooftops through the window…and they appear to be heading in their direction.

Downstairs, the Doctor tries to adapt the Vortex Interferometer so that instead of detecting disturbances in space-time, it‘s actually creating them. With any luck he can disrupt the link with 2008 to close off the corridor so the creatures can‘t maintain a presence in this time. Robert seems unusually quiet. He tells the Doctor that they’ve known and worked alongside each other for a whole year and now he’s discovered he hasn’t really known the Doctor at all. All this talk of time machines and psychic projections proves that he’s not from this world. The Doctor apologises for deceiving Robert, but his assistant is angry that he’s been lied to all this time and treated like a fool. The Doctor assures him that’s not the case, but if he’d told Robert the truth, he wouldn’t have believed it. He accepts that he’s treated Robert badly and promises to make it up to him, but Robert angrily rejects the offer and says that when this is all over he will leave the Doctor and resume his studies at the Edinburgh Medical School.

Before they can argue further, Nyssa and Thomas burst in and tell them to look outside. Robert says he can’t see anything, but the Doctor says that’s the point. They can’t see any street lamps or stars, or even the buildings opposite - only the smog which is alive with a phosphorescent light. There’s not a moment to loose and the Doctor urgently orders everyone to close every door and every window and seal them tight with damp towels or curtain strips. They have to make sure nothing can enter this building…

The group does their best to make everything airtight, but they’re not sure how much good it’s going to do. They can hear screaming coming from outside in the street and Nyssa insists on going out to help the people, but Thomas tells her there’s nothing they can do and they must already be dead. He believes the spirits are trying to trick them into opening the door. As the front doors start to rattle, Thomas grabs Nyssa’s candle and uses the wax to block the keyhole. Nyssa knows the creatures won’t give up that easily, so Thomas suggests they go downstairs to the kitchens.

The air outside is thick with the phantoms, but Robert and the Doctor are confident they’ve blocked every window on their side of the house. The Doctor decides it’s time to test the adapted Vortex Interferometer and he switches it on. Robert points out that the smoke creatures are still there, but the Doctor says the balance of probability still lies in their favour and it’ll take a while to have any effect. He thanks Robert and says he couldn’t have done this without his help, but Robert is still angry. Although there’s nothing now that can make him reconsider his decision, he‘d prefer it if they departed on good terms. The Doctor says he’s enjoyed Robert’s company over the last year and they both agree there should be no regrets. Robert still has one question to ask - why does the Doctor keep a police box in his study? The Doctor is about to explain when they hear a noise coming from the fireplace. It’s the one part of the building they forgot to block!

Thomas hears the voice of his mother inside his head again, but this time it’s very faint and it tells him something’s gone wrong with the process. Nyssa realises what’s happening and forces Thomas to concentrate on her own words. Thomas says he was simply thinking about Pickens, but Nyssa makes him promise to tell her if his mother tries to communicate with him again. The Doctor and Robert join them and tell them the creatures have found a way inside the house. Robert thinks there may still be time to block off the chimney and tries to push a table up against the flue, but he’s immediately overcome by the glowing smoke which enters every orifice in his body. Thomas grabs Nyssa and drags her to safety and the Doctor is reluctantly forced to follow them, knowing it’s already too late to save his friend. Robert sacrificed himself to give them a chance to escape and the Doctor regrets that he seems to have a habit of losing good friends. He knows the door won’t stop the creatures for long so they have to find somewhere safe. Smoke starts bellowing through the kitchen door, so the three of them cover their mouths and noses and race upstairs…

The Doctor, Nyssa and Thomas enter the upstairs bedroom with the ghostly fog creatures close behind them. Thomas points out that they can’t stay locked up in here forever, but the Doctor asks Nyssa to look out the window and report on any signs that the mist is dispersing. He explains that he’s broken their link with the year 2008 which makes their presence here a paradox as they originate from a future that cannot possibly exist. Unknown to the others, Thomas starts to hear his mother’s voice again. She tells him to destroy the Doctor’s device and she promises no harm will befall him or the others. When he returns to normal, the Doctor realises he’s been talking to his mother. Thomas blames the Doctor for bringing the demons here, then he knocks the Doctor out and escapes while Nyssa is tending to him.

Thomas makes his way downstairs, but is terrified by the spirits who seem to be everywhere. His mother tells him they won’t attack and if he proceeds into the Doctor’s study. As the smoke creatures start to fade away, Thomas approaches the Doctor’s Vortex Interferometer and his mother orders him to destroy it immediately.

The Doctor wakes up and Nyssa tells him she couldn’t stop Thomas. The mist outside is starting to clear, which means the creatures can’t sustain themselves anymore and are becoming too unlikely to exist. It seems that Earth’s future is secure. The Doctor guesses the deaths will be explained away as just another lethal London smog, which were all too frequent in this era. Nyssa wonders whether Thomas is still alive, but then they hear the sound of machinery being smashed up…

Thomas had palmed Nyssa’s key when he bumped into her a few moments earlier. His mother’s voice told him the key would open the blue box - the same box he’d seen at Creek’s shop and before then when he was a young boy. He entered the box and found himself in a room like a chapel, all stark and white. Standing there waiting for him was a woman dressed in black. She told him she would guide his hands over the console and then the room was filled with the sound of engines…

Without the TARDIS, the Doctor and Nyssa are stuck in 1867. Nyssa assumes Thomas stole the ship in order to prevent them disrupting the time corridor and tells the Doctor that the boy had recognised the blue box when he saw it earlier. He’d mentioned seeing it in a shop owned by someone called Creek and the Doctor realises this means the TARDIS is already here in London, in two places at the same time. Unfortunately the telephone directory won’t be invented for another twenty years, so it won’t be easy to track down the shop. Nyssa remembers Pickens mentioning a place called Jacob’s Island, so they decide to try there first. The smoke creatures will soon be all over London again, so they’ll have to hurry…

The TARDIS materialises. Inside the control room, Thomas’s mother has disappeared and he’s alone again. He doesn’t even know for certain whether he’s alive or dead. He has a terrible pain in his head and it’s all he can do to open the doors and go outside. He’s expecting to see the Doctor’s study in Baker Street, but there’s just darkness outside with a faint green light illuminating the remains of a city that’s fallen to rubble. Everything is burning and he can see bodies that are black and charred. There are hundreds of the smoke creatures, too many for him to count, floating around and swooping in and out of the ruined buildings as far as the eye can see.

The Doctor takes Nyssa to St Saviour’s Dock, the location on the Thames that leads to Jacob’s Island. It’s nearly dawn and across the river they can see the mist moving through the streets and deeper into the city. They haven’t much time so they carefully make their way across the slippery gangplank. The smell is overpowering and Nyssa can’t believe anyone would chose to live in such a place, but the Doctor says nobody does and it’s poverty that chooses for them. They arrive at Creek’s second hand goods shop and knock on the door, but there’s no answer. As they see the fog slowly moving across the river towards them, the Doctor has no choice but to kick the door open.

Inside the shop, the Doctor realises from the build-up of dust that the place has been abandoned for years. Everything is decaying and it seems unlikely the previous occupants left behind anything valuable, but the Doctor pulls back a sheet to reveal the TARDIS. The smoke creatures are now outside the shop, trying to find a way in, so the Doctor asks Nyssa to hand over the TARDIS key so they can leave. The truth dawns on them - Nyssa doesn’t have the key anymore because Thomas stole it from her earlier. Fortunately the Doctor always carries a spare. They let themselves in just as the smoke creatures emerge through the gaps in the door and spread out throughout the shop…

Inside the TARDIS, Nyssa asks how the ship ended up in Creek’s shop in the first place? And if this TARDIS comes from some point in their future, then where are their future selves? The Doctor says they must be around somewhere…unless something happens to them. They could even be dead. He checks the TARDIS’s memory bank and discovers that the ship has recently made two journeys. After dematerialising from the Doctor‘s study, the TARDIS was taken to the year 2008 and then it made a return trip to the year 1833. This means the ship has been waiting for them for the last 34 years. Thomas must have sent the ship into the alternative version of the future at the other end of the time corridor. It’s where the smoke creatures come from and is also the source of the psychic projection. There’s only one way to find out what happened to Thomas. The Doctor programmes the controls and the TARDIS dematerialises again.

Thomas doesn’t know how long he’s been lying on the floor of the TARDIS, half-awake and weeping for his mother. Then out of nowhere comes a wind and the sound of the TARDIS materialising. Inside their version of the TARDIS, the Doctor and Nyssa discover they’ve materialised inside the same console room. Nyssa points out that landing a TARDIS inside another is dangerous at the best of times, let alone landing inside the same TARDIS at an earlier point in its history. The Doctor thinks it’s rather neat. They go outside and find themselves back in the console room, but this time with Thomas lying on the floor. The Doctor rebukes the boy for breaking into his house, destroying a valuable piece of equipment and, last but not least, stealing his TARDIS. He demands an explanation, but Thomas just faints again. The Doctor diagnoses him severe neurological trauma as a proximity side-effect of Thomas being too close to the source of the psychic link. They look at the smouldering desolate landscape outside and realise this is one of the potential versions of 2008. The worst of all possible worlds where the smoke creatures have reduced everything to ash and clinker simply to generate the energy needed to summon this reality into existence. Thomas wakes up and the Doctor asks him where he’d seen the TARDIS before. He says he and the other boys saw it on the river not far from London Bridge, but it was covered in silt and must have been there for years. The Doctor tells Nyssa he needs to find out exactly where the TARDIS ended up in 1833 so he can programme it to travel back through the time corridor and seal it up permanently behind it, like a zip fastener. There’s no need for them to be on board so he sets a 60-second time delay and they withdraw to the other TARDIS.

Thomas is confused when he finds himself walking out of the control room and back into the exact same control room, but at a later point in time. The Doctor tries to explain and Nyssa agrees that it’s quite logical, so Thomas accepts that he’s the only one that doesn’t understand it. The Doctor makes sure this TARDIS dematerialises before the other one does, then they watch as the earlier TARDIS travels back in time, closing up the time corridor as it goes. Unfortunately the effect on Thomas is as severe as it is unexpected. His head feels like it’s about to explode and he screams out in agony. The Doctor realises that without the time corridor, the psychic link to Thomas is dying and it’s taking him with it. Then, both the Doctor and Nyssa hear the woman’s voice singing “Oranges and Lemons”. The Doctor deduces it’s a psychic test signal, which must be how the creatures tuned into Thomas’s consciousness in the first place.

The TARDIS lands and the Doctor and Nyssa emerge. They hope Thomas will be alright inside the ship on his own, but they have to locate the point where the psychic link was first established and find some way of preventing it. Events will still unfold as before, but what’s killing Thomas is the fact that his past is predicated upon a future that can never come about. Thomas built a time machine to ensure the future that gave him the information to build the time machine, and thus he created a paradox. Now they’ve removed the possibility of that future, his life has become a contradiction in causality and to save his life, they need to find some way of collapsing the time paradox to resolve the contradiction.

This is the day of Thomas Brewster’s mother’s funeral. He’s only four or five years old and is hiding in the parlour to ignore the heartless comments of his aunts and uncles who blame him for his mother’s death. The Doctor and Nyssa are watching from a safe distance and recognise the mother’s body as the same woman who appeared in the TARDIS. They spot Thomas and realise the year must be 1851, which means they’ve arrived too early.

In the TARDIS, Thomas hears the voice of his mother. She warns him that the Doctor and Nyssa are trying to prevent her from being with him by reconfiguring the past. Thomas is struggling to understand her words when the doors open and the Doctor and Nyssa return. The Doctor asks the boy when he next saw his mother after the funeral. Thomas says he doesn’t remember, but the Doctor keeps pressing him and tells him he has to accept that she’s dead. Thomas refuses to let her go and insists that she’s real, and the Doctor realises he’s clinging to her memory and is desperately trying to keep her alive, even at the expense of his own life. Nyssa remembers Thomas telling her that after the funeral he was sent to the workhouse…

The Doctor and Nyssa leave Thomas in the TARDIS again while they pop outside for a quick word with the workhouse master. The Doctor questions Shanks about Thomas Brewster and asks if the boy has mentioned anything about having bad dreams, but the man believes the visitors are wasting his time and he has them escorted from the premises. They conclude that the psychic link hasn’t yet been established so they still need to find the precise point where Thomas’s mother actually started guiding him rather than merely sending out a test signal.

They return to the TARDIS and find Thomas is deep in some sort of psychic reverie. Inside his head, Thomas has been transported outside the confines of the ‘chapel of white’ and has found himself on Southwark Bridge. It‘s the night when his mother died. He stands on the bridge for what feels like hours and then he sees his mother arrive, dressed not in black, but in her everyday housemaid’s clothes. He realises this is the moment before she jumped, but this time they’re both aware of each other’s presence. She tells him that if he saves her, she could be with him and look after him throughout his childhood. He asks her what he should do and she tells him he has to kill the Doctor and Nyssa. Thomas refuses and backs away, but his mother tells him he’s killed before when he allowed Creek, Robert and even his friend Pickens to die. She tells him he’s a killer, but he denies this and realises that if she was really his mother she wouldn’t ask him to do such a thing. He tells her he doesn’t want to know her any more and that he’d actually be better off without her. His mother becomes angry and tells him he’s the reason she killed herself - her own baby drove her to distraction and brought shame and ruination upon her. He refuses to listen, but his mind goes back to the funeral when his aunts and uncles blamed him for his mother’s suicide. He tries to block his mother’s voice out, but she continues to repeat that he could have saved her, yet he chose not to. Then she screams and jumps off the bridge into the river Thames…

Thomas returns to his senses and finds the Doctor and Nyssa watching over him in the TARDIS. Thomas realises the woman wasn’t his mother after all and that it was all a trick. He now knows he was being used by something evil. He says he used to feel bitter about his mother’s death and wished he could have done something, but now he just feels sorry for her. The Doctor asks him to remember when she first spoke to him...

The Doctor and Nyssa visit the old curiosity shop on Jacob‘s Island and Creek thinks the Doctor must be a revenue man and argues that he’s just a humble river-man…

Thomas admits that he was listening to their conversation from the back room of the shop and he heard everything they said. This was two years ago. The first contact with his mother was after Creek died and he teamed up with Pickens. The two boys run to the wharf where Creek’s personal boat is moored and head off across the river. As they row for their lives, three figures appear on the jetty and call for them to stop. The Doctor, Nyssa and Thomas try to warn the younger Thomas that he doesn’t realise the danger he’s in, but in just a few moments the three of them have disappeared from sight, shielded by the dense fog. Thomas suggests they grab another boat and try to follow them. The Doctor tells him he’ll have to say behind because the two versions of himself can’t be allowed to come into physical contact, but Thomas refuses to listen and insists on going with them.

The Doctor, Nyssa and the older Thomas begin rowing across the Thames, but it’s not long before they’re completely enveloped in the fog too. Nyssa senses that something isn’t quite right. On the other boat, the younger Thomas and Pickens start to panic as they can’t see where they’re heading. Thomas says they have to keep rowing, so they go deeper and deeper into the freezing fog. The other boat containing the Doctor’s group catches up with them and they pull alongside. The Doctor orders the older Thomas to stay in the boat whatever happens, but even from here he can see the woman in black, gliding towards them out of the mist and floating above the water. He shouts out a warning to his younger self not to listen to her - then suddenly everything goes black…

Thomas wakes up and finds himself back at the Doctor’s house on Baker Street. Nyssa offers him a cup of hot sweet tea and checks that he’s alright. The Doctor tells Thomas that he successfully warned his younger self not to listen to the psychic projection, but then he fell into the river. Thomas wonders what happened to his other self and Pickens, but then he remembers that Pickens helped him when they landed on the other side of the river at Wapping. The Doctor tells him that by warning his other self, he short circuited the paradox, causing it to collapse. In effect, he convinced the projection of its own non-existence and made it aware of its own impossibility, so it ceased to exist. Thomas doesn’t understand a word of this, but his head hurts enough as it is and he doesn’t want to hear any more. Nyssa tells him his past hasn’t changed and everything that happened to him is still part of history, but the future he saw cannot happen now. Thomas asks whether he’ll remember his mother, but the Doctor and Nyssa avoid the question and tell him to get some rest. Seconds after they’ve gone, Thomas hears the voice again. His mother tells him he’ll always remember her, but she asks him to remember her as she was before she died. She says she was so happy when he was born and he was her pride and joy. She wanted to be there for him every minute of every day, but she couldn’t do it and she hopes he’ll forgive her for leaving him. She starts to fade and Thomas pleads with her not to go. Her final words to him are that she will always love him…and then she completely disappears.

Thomas is alone once more, knowing he will never see his mother again. He lays there, thinking. He wonders what will become of him and what the future holds for him. He knows the Doctor and Nyssa will leave soon and all he has to look forward to is a life of thieving and running and hiding. Unless he does something about it.