The Doctor lands the TARDIS in Victorian England in a snowy day and emerges to stroll the streets. He speaks to a boy and confirms its Christmas 1851. A woman starts yelling "Doctor!" and the Doctor smiles and goes to investigate. He finds a woman cowering back before a door holding something back. The Doctor assures her he can deal with it but she continues to scream for the Doctor and insists there can't be two of them. Another man emerges and announces that he's the one and only Doctor. He collects his sonic screwdriver from the woman, Rosita, and tells her to go back to the TARDIS. The doors burst open and a humanoid animal wearing a Cyberman mask emerges.
Both Doctors yell "Allon-sy" and points their sonic screwdrivers at the advancing figure. The Next Doctor warns the Doctor back as the creature leaps up the wall. The Doctor concludes it's a primitive conversion and the Next Doctor grabs a rope and lassos the creature. However, it yanks him up and the Doctor grabs the rope... only to be pulled up himself. Rosita grabs an axe while the Doctor wonders if the Next Doctor knows him. The Next Doctor has no idea who he is. The creature pulls both Doctors into the building behind them and jumps out a window on the opposite side. Rosita arrives and cuts the rope just before the creature pulls them out to their deaths. They embrace in relief, laughing, and then go downstairs as Rosita complains about their sense of humor. The Next Doctor introduces Rosita and she warns they have 20 minutes until the funeral.
Now that things settle down, the Doctor assumes that the Next Doctor is his future self, but the Next Doctor doesn't recognize him. He asks who the Doctor is and the Doctor realizes something odd is going on. He uses the name John Smith and suggests that the Next Doctor is missing some memories. The Next Doctor admits there are great parts of his memory missing since his encounter with the Cybermen, now at large in London. He explains that the Cybermen found him, and then something was taken and lost. He asks the Doctor what he was like in the past but then runs off to the funeral. Once he's gone, the Doctor follows after him.
In an underground lair, the Cybermen determine that Cybershade 16, the altered creature, made contact with the Doctor. They review the recordings and confirm that the Next Doctor is the Doctor. The Cyberleader turns to a human woman, Miss Hartigan, who informs her that the plans for Ascension are underway and asks if everything is in position. She promises to do her best, operating at maximum efficiency. In return, the CyberKing assures her she'll be heralded in the new age in the court of the CyberKing. She departs for the funeral.
A funeral procession for Reverend Fairchild makes its way down the street as the Next Doctor and Rosita watch. The Next Doctor tells Rosita to go back to the TARDIS while he breaks into Fairchild's now-empty home. The Next Doctor breaks in to find the Doctor already there. The Doctor notes that the Next Doctor's "sonic screwdriver" is a simple screwdriver. As they go inside, the Next Doctor explains that Mr Jackson Lake, a teacher of mathematics, came to England three weeks ago and died a terrible death. His body was never found after that. After that there were a series of abductions and murders, mostly of children. Fairchild was the most recent murder, and someone electrocuted him. He was a pillar of the community and dealt with children's charities. The Next Doctor begins to recognize the Doctor but isn't sure how. The Doctor notices that he's wearing a fob watch and explains that the memories of a Time Lord can be contained within such a watch. The Next Doctor hands it over and the Doctor opens it out but realises it's simply a fob watch.
Using his sonic screwdriver, the Doctor secretly opens a desk and finds a metal cylinder, an infostamp. The Doctor activates one which plays back the history of London from 1066 to the present day. The Doctor realizes the Cybermen are using the infostamps because they're in the wrong century and don't have enough power for anything else. The Next Doctor remembers that he was holding such a device on the night he regenerated. He remembers the Doctor being there and demands to know who he is, and then begs for help. The Doctor agrees and says they should return to the Next Doctor's TARDIS after making one final search.
The Doctor opens a closet and finds a Cyberman inside. They run further into the house only to find another Cyberman blocking their path. The Doctor tries to hold them off with a cutlass without success and insists they need him. He yells that he is the Doctor and tells them to leave the Next Doctor alone. They continue moving up the stairs while the Next Doctor holds the infostamp and remembers triggering an energy discharge. He does so again and the Cybermen's head explodes. The Next Doctor says that he did it the last time and accuses the Doctor of trying to take away the only thing he has, just like the Cybermen did. However, he can't remember what they took. The Doctor promises that they'll find out together.
Fairchild is buried at the cemetery as Miss Hartigan arrives in a bright red dress. Everyone is shocked to see her and accuse her of being a whore. She explains that she's the Matron of the St. Joseph Workhouse and explains that Fairchild had to die so that they could all be gathered in one place. The Cybermen arrive, marching between the tombstones, Miss Hartigan selects four men for selection and informs the others that they are disposable. The Cybershades and Cybermen kill the others. The selected men wonder why they were spared and Miss Hartigan explains that their children are necessary.
The Doctor and the Next Doctor arrive at the latter's TARDIS, stored at a stable. Scattered about is the luggage that belonged to Jackson Lane, the first man who died. The Next Doctor admits that the Doctor was braver than him. The Doctor scans about with his sonic screwdriver and then opens one of the trunks. As he goes through it, Rosita explains that the Next Doctor saved her from a Cyberman and that he has bad dreams. Inside the trunk, the Doctor finds another infostamp. The Next Doctor takes him to the TARDIS... a hot-air balloon. The Next Doctor pays his assistant Jed and promises that it's not quite ready, but one day he will ascend. He says that he will only depart when London is safe, and then he will have a perfect escape. The Doctor asks if he wants to know what he's escaping from, and says he's worked everything out. The Next Doctor considers the matter without responding.
The four chosen men have been equipped with cyber adapters. Miss Hartigan tests to make sure they will obey her commands and then tells them to bring her their children. She then rides off into the night with a Cybershade as her driver.
The Doctor explains that the Cybermen were defeated and sent into the Void, and then everything inside the Void perished. The last of the Cybermen fell back in time. At the same time, Jackson Lane came to London with plenty of luggage and money. He found the Cybermen too, and took hold of an infostamp. When the Next Doctor points out that Jackson Lane was killed, the Doctor notes that he never opened the luggage. On the Next Doctor's watch are the initial JL. They realize that the "Next Doctor" is Jackson Lane. The infostamp he picked up contained information on the Doctor. It backfired and loaded the information into Jackson's mind. Jackson remembers what happened and how he absorbed all of the data, and realizes that he's lie. The Doctor insists that the bravery and the inventiveness is Jackson's, but there's still something missing. When Jackson demands to know what else he's missing, the Doctor explains that he's suffering from a fugue state brought on when Jackson lost something vital. As the clock chimes midnight, Jackson remembers that the Cybermen killed his wife Caroline.
The two infostamps start to signal and the Doctor hears the activation sound from a nearby trunk. It contains numerous infostamps and the Doctor realizes the signal means the Cybermen are on the move. He runs out into the street and Jackson tells Rosita to go with him. She joins the Doctor and they watch as Mr. Cole, the master of a workhouse, takes his youthful charges down the street. The Doctor tries to talk to Cole, who ignores him, and hesitates when he realizes the Cybershades are watching and will attack if he interferes. Jed informs them that he's seen other children being escorted through the streets. The Doctor, Rosita, and Jed follow the children as they're taken to a sewer entrance and the Cybermen and Cybershades emerge to escort them to the court of the CyberKing.
In the stables, Jackson goes through his luggage looking for one item.
The Doctor realizes they need another way in. As he circles around, Miss Hartigan and two Cybermen block them. She insists that she isn't converted and she chose to aid the Cybermen in return for liberation. The Cybermen note that the Doctor doesn't correspond to their image and he explains that their database was corrupted. He tosses them the infostamp and suggests they use it. They realize that it's been damaged and would harm them if used unmodified. They repair it and access the data, and realize they're facing the real Doctor. The Doctor asks why they need the children and Miss Hartigan explains that they will serve as a workforce, and she chose Christmas Day for a new birth. She tells the Cybermen to delete the Doctor. As they move in, Jackson fires an energy beam from one of the infostamps, destroying them. Miss Hartigan calls the Cybershades and Rosita slugs her before fleeing. As they escape, Miss Hartigan tells the Cybershades to inform their masters that the CyberKing will rise tonight.
As they run, Jackson explains that he found the deed to the house he and his wife were moving to. They discovered the Cybermen there in the cellar and the Doctor realizes they can gain access to the Cybermen's sewer base from there. However, Jackson insists there was something else in the room that he can't remember, something that may be the key to stopping the invaders.
The Cyberleader agrees with Miss Hartigan's opinion that the Ascension should begin immediately. She kills the four workhouse owners and then tells the children that the new Industrial Revolution will begin. They go to work and the Cyberleader confirms that their power levels are rising. She asks to see the CyberKing and he takes her to it.
In the house cellar, Jackson destroys a Cyberman and the Doctor finds the source of their time travel ability: a dimension vault. It's not the item that Jackson is trying to remember.
The Cyberleader takes Miss Hartigan to the CyberKing's throne. When she wonders who will sit there, it turns to her. She reminds the Cyberleader of his promise and it informs her that it lied.
As the Doctor, Rosita, and Jackson make their way through the sewers, the Cybermen place Miss Hartigan on the throne and inform her she will be liberated from all of her emotions. A conversion helmet is placed on her and the Cybermen herald her as their new ruler.
The trio arrive in the sewer foundry and realize that the children are building a vast engine that generates electricity. The Doctor realizes that the software in the power core is rewriting itself. On the throne, the CyberKing says it can see the whole of infinity and declares it glorious. The Cyberleader says that "glorious" is an unacceptable emotion result and Miss Hartigan explains that her mind is powerful enough to resist conversion. She announces there will be a new race of Cybermen, logic and strength combined with fury and passion. They move in to remove her and Miss Hartigan destroys the Cyberleader.
The Doctor determines that the software rewrite is almost complete and the children will be destroyed when the power reaches 100%.
The Cybermen declare their allegiance to the new CyberKing and prepare to delete their workforce. Hartigan commands them to come to her. The Doctor and the others arrive and Jackson and Rosita use the infostamps to destroy the Cybermen. Rosita gets the children out while the Doctor determines the engine is some kind of starter motor. As the children escape, Jackson looks at them and remembers his own son being taken away as his wife was killed. He sees his son Frederic at the top of the engine tower. The boy is too scared to move and the flames destroy the stairs up before they can get to him. The Doctor grabs a cutlass and cuts a rope, rising up into the chamber. He grabs Frederic and swings to safety just in time.
As everyone flees the building, Rosita looks on in horror as a vast CyberKing arises from the Thames. The Doctor grabs the dimension vault and emerges from the cellar, and explains that the CyberKing is a dreadnought class ship, the first line of an invasion. A Cyberfactory is inside its chest, capable of converting millions. Hartigan orders the CyberKing to stride across the city, destroying everything in its path. The Doctor tells Jackson to get Frederic to safety and then goes to stop the CyberKing. He returns to the stable and grabs the remaining infostamps, and then tells Jed to prepare the TARDIS balloon for flight.
The TARDIS rises into the sky behind the CyberKing and the Cybermen's proximity alert goes off. Hartigan orders the CyberKing to turn and prepares to destroy the Doctor. He offers to use the dimensional vault to seek out a world where they can live in peace. Hartigan turns down his offer, insisting the world below is sufficient. The Doctor activates all of the infostamps and fires the data blast into Hartigan. She is unimpressed and unaffected, but he explains that all he did was break the cyberconnection. Her mind is now open for the first time in many years. The Doctor tells her to look at herself and face what she's done. She screams and the CyberKing explodes, taking her and the Cyberforces along with her. The CyberKing starts to fall and everyone runs in panic. The dimensional vault reaches full charge and the Doctor fires it, transporting the CyberKing away into the time vortex before it can fall and destroy London. On the ground, Jackson rallies the people to cheer their thanks to the Doctor.
Later, Jackson accompanies the Doctor to his TARDIS and says that a new history will begin for him as a widower with a son and a good friend in Rosita. He invites the Doctor to join them for a Christmas feast in memory of those they have lost. However, he realizes, the Doctor won't stay. Jackson is amazed at the sight of the TARDIS and asks if he can look inside for one last adventure. Jackson runs out as his head starts to ache with the sights. They say their goodbyes and Jackson asks why he doesn't have a companion. The Doctor admits that many have come and they all break his heart. Jackson insists that he stay for Christmas Dinner and the Doctor agrees.