The Doctor lands in London on Christmas Eve, 1851. Hearing cries for help, he comes across a man who calls himself the Doctor (David Morrissey) and his companion Rosita Faris (Velile Tshabalala). They are trying to capture a Cybershade, a kind of shaggy ape with a Cyberman head. The man suffers from amnesia and the Doctor thinks he might be a future incarnation of himself. The Next Doctor takes the Doctor to a house nearby, home of a recently deceased minister, as he thinks the minister is connected to a series of disappearances in London. Inside, they discover a pair of Cyberman data-storage infostamps. The Next Doctor recalls holding them the night he lost his memory.

They return with Rosita to the Next Doctor’s base of operations. The Doctor realizes the man is not a Time-Lord, but a human being named Jackson Lake, one of the missing people. The Doctor thinks Lake encountered the Cybermen and warded them off with the Cyberstamps, absorbing knowledge about the real Doctor. Entering a fugue state at the shock of seeing the Cybermen kill his wife, he came to believe he was the Doctor. The Doctor and Rosita try to find the source of the Cybermen.

They find many children, pulled from workhouses throughout the city by Miss Mercy Hartigan (Derula Kiwan) and put to work at an underground facility guarded by Cybermen. The Cybermen turn against Miss Hartigan and convert her into the controller of the CyberKing, a huge robot, like the Jaegers in Guillermo del Toro’s Pacific Rim, powered by the energy of the children. She also gains control of the Cybermen.

Jackson remembers finding the Cybermen while moving to his new home. The Doctor finds another entrance to the base under Jackson’s house. As the CyberKing starts to rise into the City, the three rescue the children, including Jackson’s son. The CyberKing stomps through London like a steampunk Mecha-Godzilla. Miss Hartigan refuses the Doctor’s offer to leave the planet. The Doctor cuts her connection to the CyberKing with the infostamps, exposing her to what she has done. Both Miss Hartigan and the CyberKing are destroyed. The Doctor pulls the CyberKing into the Time Vortex, saving London. The two Doctors have Christmas dinner.

David Tennant had already announced that he was leaving in 2010. He was busy playing Hamlet in 2008 and 2009, so the Doctor Who people came out with five hour-long specials featuring him and no companion. This episode was the first, and a Christmas special. It was released to excellent reviews and nominated for a Hugo but lost to The Waters of Mars, another special. This was the last story to be produced in standard definition. Jackson Lake’s wardrobe and haircut resembled that of the 11th Doctor, coming up. The Next Doctor’s TARDIS was a hot-air balloon called a Tethered Aerial Release Developed in Style. Rosita was a combination of Rose and Martha, with a little Donna thrown in. Mercy Hartigan was a dark villain coming out of a life of Victorian abuse and exploitation. The images of all ten previous doctors appeared, including Paul McGann from the TV-movie, fixing it in canon.