3 December, 1952. East End gangster Tommy Ramsey emerges from a six-month stretch in Wandsworth Prison to learn that things have been going downhill in his absence. A watchmaker on Old Street is refusing to pay protection money. Takings are down due to the teachings of Father Xavier Simmons, an American priest at St Luke’s Church. And a new gang of young toughs, led by a troublemaker named Callum, is trying to muscle in on the Ramsey Gang’s territory.

Tommy’s lieutenant, Jack Cooper, has captured one of Callum’s gang, and Tommy throws him out of a moving car as a warning to Callum. He then sends Billy Valance to the watchmaker’s shop, Fixing Time, to knock some sense into the old watchmaker. But Valance gets more than he’s expecting, for the old watchmaker is in fact the Doctor. While researching an article, Sarah Jane Smith learned that thousands of Londoners were killed by toxic smog in early December, 1952 -- and she also found a photograph of the Doctor shaking hands with Tommy Ramsey in front of St Luke’s Church. Something in the background of the photograph intrigued the Doctor, who took Sarah back to 1952 to prevent an even worse tragedy. The Doctor has opened the watchmaker’s shop in order to pass unnoticed in the neighbourhood while he investigates, and when Valance attempts to rough him up, the Doctor responds with a brief but effective demonstration of Venusian aikido.

Sarah goes undercover as a waitress at the Red Room, where takings are particularly low -- and when Tommy arrives to question his manager, Max Morgan, Sarah reveals that he’s been skimming from the top and keeping two sets of ledgers. She claims that she wants to move up in Tommy’s organisation, and, impressed by her honesty and willingness to stand up to him even though she’s clearly terrified, he tells her to visit his home on Tabernacle Street the next day. Before leaving, Sarah advises him to let Max live, as he’ll be too frightened to betray him again. Tommy has his hulking enforcer, Arthur “Brick” Baldwin, follow Sarah home to ensure that she’s genuine, and he departs satisfied when she enters the boarding house run by Rose Kelly and her husband Frank. As soon as she’s sure it’s safe, however, Sarah slips out and returns to the TARDIS, which is parked in the alleyway behind the Doctor’s shop. Shaken by her close shave with Tommy, she now realises just what it means that thousands of people are about to die -- but the Doctor insists that he can do nothing to avert the catastrophe, which is part of established history. All he can do is try to prevent something even worse from happening.

Tommy is at home, eating a dinner prepared by his mother Vera, when his police informant Bob Valentine arrives; Valentine fell under Tommy’s thumb due to his gambling debts, and is now an alcoholic with no self-respect whatsoever. He tells Tommy where Callum’s gang hangs out, and Tommy decides to send his man Jim Harris to keep an eye on the troublemakers. Valance then arrives, dazed, and the furious Tommy orders him to return to Fixing Time tomorrow with Brick and teach the watchmaker a more permanent lesson. But Callum’s gang is the real threat, for Callum seems to enjoy chaos for the sake of it, and when he learns that one of his boys was thrown out of a moving car he whips the others up into a frenzy, demanding blood. Today the Ramsey gang hurt of them; tomorrow, they will kill one of the Ramsey gang.

4 December. Father Simmons invites Chief Superintendent Derek Carver to open the first of Simmons’ Bread of Life retail outlets. Here, local people make bread which is sold cheaply in the neighbourhood, providing the community with employment and food. Impressed, Carver agrees to let Simmons supply all of the police canteens under his jurisdiction. Meanwhile, Callum catches Jim Harris staking out his hideout, slices him open and lets him stagger away to his death. Harris makes it to the steps of St Luke’s before collapsing, and Father Simmons tries to tend to him -- until he sees Callum’s horribly familiar face in the crowd. As Harris is taken to the hospital, Simmons tries to speak to Callum and warn him that his violence is undoing all Simmons has worked for -- but Callum simply scoffs. Six years ago, while still an ordinary American GI, Xavier Simmons offered to sell contraband cigarettes to a black marketeer, only to murder and rob him on the steps of the church. Simmons claims to have seen the light and mended his ways, but while he claims to have been reborn that night, Callum knows what kind of person Xavier Simmons really is.

Sarah’s arrival on Tabernacle Street draws curious stares from the neighbours, including single mother Mary Mills. Sarah has tea with Vera while she waits for Tommy, who gives her the oddest and most terrifying job interview of her life. He eventually decides to keep her by his side for the next few days on probation. Much to Tommy’s surprise, the Doctor then arrives to speak with him, having overpowered Valance once again and hypnotised Brick. The Doctor tries to convince Tommy that there are greater threats to the East End than he knows, and that his racketeering and protection are only making matters worse. However, Tommy insists that he and his men protect the people on their turf, and informs the Doctor that while he’s been talking, Jack Cooper has gone around to his shop and burned it down.

Tommy sends Brick to the Kellys’ to collect Sarah’s belongings, and Frank Kelly, who once tried to peep on Sarah in her bath, goes pale when he realises she’s apparently Tommy Ramsey’s girl. Meanwhile, Jack burns down Fixing Time and then reports to his real boss -- Tommy’s rival Steve MacManus, who has agreed to let Jack take over Tommy’s turf once Jack disposes of Tommy. Jack then meets Brick and returns to Tommy’s, just as Bob Valentine informs him that Jim Harris is dead. Infuriated, Tommy orders Sarah to stay at the house tomorrow with Vera while he and his men deal with Callum’s gang once and for all.

5 December. The rival gangs set off for St Luke’s Church. Sarah stays at Tabernacle Street, where she chats separately with Mary Mills and Brick. Mary is a single mother who has somehow found the strength to raise three illegitimate daughters in the moral climate of the 1950s, while Brick is a gentle man whose size is usually enough to deter people; he is upset whenever he must resort to violence, and he keeps pigeons on the roof. Elsewhere, a fresh-faced young constable, Andy Hodge, is warned by Sergeant Diggle to keep away from St Luke’s and let the gangs fight it out, but Hodge nevertheless vows to be there and do what’s right.

Xavier Simmons finds the Doctor sadly studying the smoking ruins of Fixing Time, and they chat about the violence in the East End, Simmons’ work in the community and his belief in the word of the Saviour. Their conversation is interrupted by the arrival of the two gangs. Though violence seems inevitable, Simmons attempts to intervene, but Callum pushes him away. Tommy then strikes the first blow, severing Callum’s arm with a sword, but Callum simply laughs -- and sheds his human guise, revealing that he is in fact an alien Xhinn. The creature that was Callum begins firing energy bolts from its body, indiscriminately slaughtering both its own men and Tommy’s.

Tommy is forced to retreat, taking the severed arm with him, while the Doctor and Simmons take shelter in the church. Struggling to understand what’s happening, Simmons wonders if he’s seen a fallen angel; the Doctor, however, knows the Xhinn by reputation as a race of conquerors who invade planets and strip them of their resources in order to fuel further expansion. The Earth is in terrible danger, but now the Doctor knows his enemy and can prepare a defense. He slips back to the TARDIS while the Xhinn is distracted; Hodge has arrived only to be attacked by the Xhinn, who taunts him, deliberately chasing him from the scene in terror but allowing him to live. Hodge flees back to the police station and babbles his story to Diggle, who is bemused but investigates nonetheless. When Simmons talks about fallen angels, however, Diggle decides not to report the incident; even Hodge has calmed down now, and is beginning to wonder whether he imagined it all.

The Xhinn Triumverate, the gestalt leaders of this scouting mission, detect the disturbance and realise that one of their agents has gone too far. They thus force Callum to return to their ship and strip him of his life essence, absorbing it back into themselves. Callum was one of the agents they sent out to walk among the people of Earth, learning their ways and preparing for the arrival of the main invasion fleet -- but he became corrupted by human ways, and now he’s revealed their presence before they’re ready. The Xhinn must take steps to correct this. As the Doctor works inside the TARDIS, he detects a nearby energy surge, and soon afterwards, a change in the composition of the outside atmosphere...

Two of Callum’s gang, Billy and Charlie, have survived the slaughter and now realise that they were on the wrong side all along. Determined to fight the real enemy, they decide to throw in their lot with Tommy after all. Tommy and his gang, however, are unsure how to respond to this alien threat. Sarah’s suggestion that they call the police is met with scorn, and in any case, they soon have no proof, as Callum’s arm suddenly tries to throttle Tommy and Brick throws it in the fireplace. Tommy decides to contact his rivals and try to convince them to join forces, and Jack manages to persuade Steve MacManus to meet with Tommy tomorrow. With this threat in the foreground, nobody but Sarah notices that the smog is getting worse. A thermal inversion has trapped a layer of cold air near the ground, creating a heavy fog which has combined with the smoke from thousands of coal fires in the East End. Bronchial and asthmatic complaints and car crashes cause the hospital to begin filling up, and worse is to come...

6 December. Dawn breaks over London, but the smog is too thick for the sunlight to show through. While Tommy searches for some of his men who haven’t yet reported in, Sarah makes an excuse and leaves to visit the Doctor; on her way out, she chats with Mary, who is concerned for her youngest daughter’s health. By the time Sarah reaches the TARDIS her face is caked with soot -- the same soot which is getting into people’s lungs all over the city. The Doctor suspects that the atmospheric disturbance is a first-strike weapon called into action by the Xhinn to prevent the events of yesterday from revealing their presence; thousands will die as a result, but the Doctor hopes to convince the Xhinn to leave before the tragedy becomes even worse. He sends Sarah back to Tabernacle Street with a warning; the Xhinn sometimes have unwitting collaborators, and not everyone can be trusted.

The Xhinn detect another alien presence nearby, and activate their drones in response. At the Bethnal Green police station, Sergeant Diggle sends his men out to evacuate the neighbourhood, claiming that no wireless announcements have been made for fear of causing a panic. Frank and Rose Kelly are amongst the first to be evacuated, but although they enter the van willingly with the rest of their neighbours, they begin to become uneasy when the van dips down into a large underground cavern. The policemen herd the Kellys and their neighbours into a relatively small chamber -- and then begin herding more and more people into the room, until the crush becomes suffocating...

Billy and Charlie manage to convince Tommy that they’re willing to fight alongside him, against the thing they thought was Callum. Unfortunately, Tommy’s rivals are harder to convince of the truth. Steve MacManus walks out on him, ignoring even Jack’s attempts to convince him that the bizarre story is true; however, as MacManus heads back to his home turf, he’s confronted by Bob Valentine, who shoots and kills him. Sarah, on her way back, runs into Hodge, who tries to evacuate her -- and doesn’t listen to her attempts to explain that she lives somewhere else. When she sees policemen beating people who refuse to leave their homes, she overpowers Hodge and flees back to Tabernacle Street with her story. However, Tommy doesn’t have enough men to act against the police, especially as many of his most trusted men have gone missing. He’ll have to wait until morning, even though that means leaving innocent people to the possessed policemen. If Fingers Blake and Stratford Simon don’t come through for him, he and his men will have to do what they can alone.

The Doctor traces the source of the weather disruption to the Xhinn ship, where they capture and interrogate him. He’s horrified to see that the brainwashed policemen have been cramming more people into the holding chamber than it can contain, but he can do nothing to save them. Rose and Frank Kelly are killed in the crush, and the Xhinn then release poison gas into the room, killing those who had survived until then. The bodies are placed on a conveyer belt, and the policemen strip them of their clothing before sending them on to be processed into a food source for the Xhinn. The Doctor, revolted, nevertheless tries to reason with the Xhinn; when this fails, he threatens to destroy them if they don’t leave at once. They conclude that he’s bluffing, and torture him until he agrees to reveal the secrets of time travel. Their control over the policemen slips while they concentrate on the Doctor, however, and as the Doctor leads one of them out to his TARDIS, Sergeant Diggle leads an attack on it. The Doctor manages to escape, but the Xhinn recovers from its surprise, kills Diggle and sends the other policemen, including Hodge, back to work on the line. As punishment for the attack, it leaves them fully aware of what they’re doing, but unable to control their bodies.

Sarah finds Brick crying on the roof, surrounded by the bodies of his pigeons. Mary Mills’ daughter Bette also dies, unable to breathe the toxic air, and the reality of what’s happening hits Sarah for the first time. Sarah warns Mary not to go with any policemen if they come to evacuate her, and returns to Tommy’s house, where she’s disgusted when Vera casually mentions that the unbaptised and illegitimate Bette will be going to hell. Tommy is suffering from his inability to act until he has enough men; however, he’s determined to strike against the aliens tomorrow, even if he can only make a futile gesture. Seeing his vulnerability, Sarah finally admits the truth about herself and the Doctor, and shows Tommy the photograph of himself and the Doctor shaking hands in front of St Luke’s Church.

Simmons prays for guidance to the Saviour, and calls his parishioners, inviting them to an emergency service tomorrow. The Doctor arrives, seeking someone to talk to about his moral dilemma; he can defeat the Xhinn, but only by resorting to their methods. Simmons admits that he once killed a man before seeing the error of his ways, and advises the Doctor to do whatever he believes is right. The Doctor thus returns to the TARDIS, where he finds that London’s air will be entirely poisonous within a day’s time. Meanwhile, Simmons heads out to lend aid to the suffering, and sees a police van transporting screaming people to the Xhinnship. He rushes after it, but arrives too late to prevent the Xhinn from gassing their latest victims. The Xhinn transport him away as the possessed policemen carry the bodies to the conveyer belt. In the TARDIS, the Doctor begins building a terrible device, knowing that he now has no choice but to use it.

7 December. Simmons awakens to find himself in the Xhinn mothership, where he realises that the light of the Xhinn is the same light he saw six years ago. All this time he’s been serving a false god, unwittingly paving the way for the Xhinn invasion. Even the Bread of Life is a cruel trick, for it contains a drug which weakens people’s resistance to mind control; the policemen carting innocent victims to the gas chambers fell under the Xhinn’s control because Simmons convinced Carter to let him supply bread to the local police station canteens. Worse, the triumverate then reveals that Simmons himself is a Xhinn; the real Xavier Simmons died six years ago, but his replacement has lived too long amongst humans and has forgotten the truth of his origins. Simmons refuses to serve the Xhinn any longer, and the triumverate thus strips him of his life essence and goes on to “greet” the people who have assembled for his emergency service... including Vera Ramsey and Sarah.

No reinforcements arrive at Tabernacle Street, so Tommy and his men take the battle out to the streets by themselves. At first Tommy urges his men not to kill the policemen unless they have to, but he soon realises that the policemen already dead, their bodies reanimated. There are too many zombies for Tommy and his men to handle, and they’re forced to retreat to Tabernacle Street, where the Doctor is waiting for them. He’s finished his weapon, but needs Tommy’s help to get close enough to use it. He has deduced that the Xhinn motherhship is located beneath St Luke’s Church, which explains what he saw in the photograph -- a warpshadow ripple in the structure of the building, evidence that a weapon beyond human understanding had been used in the vicinity. He has also seen the policemen dragging people from their homes, and has deduced what’s happening to them from his earlier conversations with Simmons.

Vera returns to the house, claiming to have been separated from Sarah on her way back, but when the dead policemen surround the house, trying to break in, Vera suddenly attacks her son with knitting needles. The Doctor knocks her out, deducing that she ate enough of the contaminated bread to fall under the Xhinn’s spell -- but that she still hesitated to kill her own son. Billy and Charlie tie her up, and after the Doctor assures him that the police will regard her as another drone and thus leave her alone, Tommy and his men make a break for it. Jack lures the police out through the back door while the Doctor accompanies Tommy, Billy and Charlie over the roofs. Eventually, Jack is cornered by the dead policemen, but he sets himself on fire and throws himself at them. Meanwhile, the police continue to “evacuate” the neighbourhood, and when they come for Mary, she tells her daughters Jean and Rita to hide. She leaves with the police of her own free will, carrying Bette’s body and claiming that only the two of them live in the house. Thus, she dies in the gas chamber, but her remaining two daughters survive.

Bob Valentine catches up with the Doctor and his allies near St Luke’s Church, and tries to kill them; he too is being controlled by the Xhinn, but they’re feeding his desire for revenge against the gangsters who used him by sending him out to kill anyone he thinks might pose a threat. This is why so many of Tommy’s men have gone missing, and presumably why Fingers and Stratford never got back to him after the meeting. The Doctor overpowers Valentine, saving Tommy’s life, but to his disgust, Tommy then shoots Valentine dead. They continue on to St Luke’s, but there, they hear people screaming from inside the Bread of Life factory -- and hear Sarah screaming inside the church. The Doctor sends Tommy, Billy and Charlie to rescue the victims in the factory and shut it down, and tells Brick to stand guard outside the church so he can save Sarah without having to worry about covering his back.

Tommy, Billy and Charlie attack the factory, where zombie policemen are forcing slaves to work on a production line; whenever somebody collapses, exhausted, the policemen fling the body into a machine which tears it apart, even if the person is still alive. The three gangsters manage to free most of the surviving slaves, but are then surrounded by zombies. They’re running out of ammunition, but Charlie has the bright idea to throw drums of chemicals at the zombies and then shoot the drums, blowing them up and setting the zombies on fire. The fire spreads to the other drums, but Tommy, Billy and Charlie escape moments before the factory explodes.

The Xhinn have been torturing Sarah while they wait for the Doctor to arrive, having used Vera and Sarah to lure him here. However, he’s brought a Time Bomb with him, a weapon which has been outlawed by every civilised race in the galaxy but which is now his only recourse. If the Xhinn do not abandon their invasion and leave Earth in peace, the Doctor will use the bomb to accelerate Time and destroy them and their ship. The Xhinn call his bluff, knowing that he won’t risk killing Sarah by detonating the bomb in her presence -- but then Tommy and Brick enter the church, distracting the Xhinn long enough for the Doctor to activate the bomb. Tommy shoots the pipe which supplies natural gas to the church’s lamps, causing them to explode; one of the Xhinn is killed in the explosion, and the shock causes the other two to lose thier hold over the possessed policemen. Sickened by what they’ve done, the policemen help the surviving victims of the Xhinn to escape from the ship. The Doctor, Tommy, Sarah and Brick flee from the church, but the Xhinn fire wildly after them and kill Brick before he can escape. The Time Bomb then explodes, and the two remaining Xhinn are caught in the accelerated Time field and killed. The Time Bomb burns out after destroying the Xhinn ship, and the Doctor destroys it to ensure that it can’t be used again. However, too many people have died for him to count this as a victory.

8 December. The smog is clearing up, but thousands are dead and those who survived the horrors don’t want to talk about them. PC Hodge shoots himself, unable to live with the memory of what he was made to do. Sarah, upset by Brick’s death, is even more upset to learn that Mary Mills is dead as well; however, Tommy assures her that Vera will care for Rita and Jean. Reporter Terry Sharp and photographer Bob Cohen arrive to investigate the explosion at St Luke’s, and Cohen snaps a photograph of the Doctor and Tommy shaking hands before the church. The building now bears the characteristic ripples of a warpshadow, the same structural anomaly which attracted the Doctor’s attention when he first saw the photograph. The Xhinn spearhead has been defeated... but the Doctor knows that this was only a scouting mission, and the main invasion fleet may still be on its way. And so does Xavier Simmons, who was only stripped of his Xhinn life force. He will walk the Earth as a human for the next fifty years, growing older, waiting to find out if there really is a signal beacon hidden somewhere in the condemned church -- waiting to see if the main Xhinn fleet really will arrive on Earth in the early years of the 21st century...