Doctor: David Tennant (Tenth Doctor)
Companions: David Morrissey (Jackson Lake)
Velile Tshabalala (Rosita Farisi)
Dervla Kirwan – Miss Hartigan
Nicholas Briggs – Cybermen voices
Paul Kasey – Cyberleader
Ruari Mears – Cybershade
Edmund Kente – Mr Scoones
Michael Bertenshaw – Mr Cole
Jason Morell – Vicar
Neil McDermott – Jed
Ashley Horne – Lad
Tom Langford – Frederic
Jordan Southwell – Urchin
Matthew Allick – Docker
Directed by Andy Goddard
Written by Russell T Davies
Script editor Lindsey Alford
Produced by Susie Liggat
Executive producer(s) Russell T Davies
Incidental music composer Murray Gold
Production code 4.14
Series 2008–2010 specials
Length 60 minutes
Originally broadcast 25 December 2008
The Doctor lands his TARDIS in London on Christmas Eve, 1851. Overhearing cries for help, he encounters a man calling himself "The Doctor" and his companion Rosita, attempting to capture a Cybershade. The Cybershade escapes the trio. The Doctor, in talking to the man, comes to believe he may be a future incarnation of himself, who is suffering from amnesia. The man, dubbed the Next Doctor, takes the Doctor to a nearby house of a recently deceased reverend, believing him tied to a series of disappearances around London and the Cybershade. Inside, they discover a pair of Cybermen data-storage infostamps, which the Next Doctor recalls holding the night that he lost his memories. The two are attacked by Cybermen and the Doctor attempts to fight them off with a cutlass, but the Next Doctor kills them using electrical discharge from the infostamps.
The two Doctors regroup with Rosita at the Next Doctor's base, where the Next Doctor claims his "TARDIS" is located. The Doctor is surprised to find that "TARDIS" is a gas balloon - "Tethered Aerial Release Developed In Style", and comes to realise that the Next Doctor is really a human, Jackson Lake, the supposed first missing person. The Doctor suspects that Jackson had encountered the Cybermen and used the infostamps, containing knowledge of the Doctor, to ward them off. Jackson's mind then entered a fugue state from the trauma of the Cybermen killing his wife, and as the infostamp had infused his mind with knowledge of the Doctor, he came to believe he was the Doctor. As Jackson contemplates this revelation, the Doctor and Rosita set off to try to find the source of the Cybermen, while the Doctor theorises that they have somehow managed to escape the Void using a Dimension Vault stolen from the Daleks (episode: "Doomsday").
The Doctor and Rosita enter an underground complex to find numerous children, pulled from workhouses around the city, at work under Cybermen guard. They encounter the bitter Miss Mercy Hartigan, the Cybermen's human ally that has brought the children to them for labour. The Doctor attempts to use a corrupted infostamp to overload the Cybermen's systems, but they instead repair it and identify the Doctor as their long-time foe, and prepare to "delete" him and Rosita. Jackson suddenly arrives, armed with several more infostamps which he uses to distract the Cybermen long enough for the three to escape. The Cybermen turn on Miss Hartigan, converting her into the controller for the "Cyberking", a giant mechanical Cyberman powered by the energy generated by the children. She tries to protest, saying that the Cybermen promised her she would never be converted, but the Cyberleader claims "That was designated a lie."
Jackson explains to the Doctor how he has started recovering his memories, and remembers encountering the Cybermen on moving into his new home. The Doctor considers that Jackson's home may be close to the Cybermens' base, and discovers a second entrance there. Within the complex, as the Cyberking starts to rise to the city, the three rescue the children, including Jackson's son, who was abducted in the initial attack that triggered Jackson's fugue state. As the Cyberking starts to lay waste to the city, the Doctor uses Jackson's balloon to rise near the level of the Cyberking's control room in the machine's head, and tries to reason with Miss Hartigan, offering to take her and the Cybermen to a new planet. When she refuses, the Doctor uses the infostamps to sever her connection to the Cyberking, exposing her to the raw emotion of what she has done. Enraged by the actions the Cybermen forced her to undertake, the emotional feedback destroys both the Cybermen and Miss Hartigan. As the Cyberking starts to topple, the Doctor uses the Dimensional Vault to draw both it and the remnants of the Cybermen into the Time Vortex, saving London. The crowds of people below, rallied by a speech by Jackson, cheer and applaud the Doctor.
In the aftermath, Jackson thanks the Doctor for what he has done and is allowed to see the interior of the true TARDIS, much to his pleasure. He then offers the Doctor a place at Christmas dinner. The Doctor initially refuses the offer, but is convinced when Jackson becomes more emphatic. Before they depart, Jackson enquires after the Doctor's many companions, and the Doctor replies that in the end, "...they break my heart," they move on, and he is left alone. The pair then head off for a Christmas dinner in honour of those they have lost.