At Fairchild's funeral, Mercy Hartigan and the Cybermen attack the mourners, sparing four who are subsequently fitted with Ear-Pods and dispatched by Miss Hartigan to their workhouses to recruit the children. Returning to the Next Doctor's home base, the Tenth Doctor is shown the other's TARDIS "Tethered Aerial Release Developed In Style" - a gas balloon. Realising what has happened, the Doctor explains that the Cybermen have escaped from the Void (following the Battle of Canary Wharf) when the walls of the universe were weakened in "a greater battle". The Cybermen came upon a man named Jackson Lake, the first person to disappear, attacking him and his wife. In the confusion, Lake destroyed the Cybermen with an infostamp (one containing information on the Doctor gleaned from the Daleks), as earlier in the house, but it also backfired, overwhelming Lake's mind with information about the Doctor. In despair at losing his wife, Lake entered a fugue state and came to believe he was the Doctor. Meanwhile, the children are taken to a sluice gate to the Thames. The Doctor and Rosita investigate and are confronted by Miss Hartigan, who explains that the Cybermen offered her liberation. The Doctor returns the infostamp to the Cybermen, who download it, confirming him as their foe. Miss Hartigan orders the Cybermen to delete the pair, but Lake appears and destroys the Cybermen with another infostamp, allowing them to escape. A furious Miss Hartigan announces that "the CyberKing will rise tonight!"
Lake reveals that he and his family were attacked at their new house and the Doctor realises it may lead to the Cybermen's base. There, they find a Dimension Vault, stolen Dalek technology that allowed the Cybermen to escape the Void. In the Cyber-base, the captive children are working to generate power to allow the CyberKing to ascend. Hartigan is betrayed by the CyberLeader and 'converted' to the CyberKing - thus receiving liberation from her anger and hatred. However, she proves too powerful to control, and uses her new powers to destroy the CyberLeader. The Doctor, Rosita and Jackson evacuate the children, including Jackson's son who was abducted when he was attacked. However, the CyberKing - a giant Cyberman-shaped robot ship - emerges from the Thames and begins to lay waste to London. Using the gas balloon, the Doctor confronts Hartigan and offers her a chance to live in peace. When she refuses, the Doctor uses the infostamps to sever her connection from the CyberKing. Realising what she has become, Hartigan screams in horror destroying the Cybermen and herself. Before the CyberKing can collapse on the city, the Doctor uses the dimension vault to transport it into the Time Vortex. In the aftermath, Jackson thanks the Doctor for what he has done and offers him a place at his Christmas celebration with Rosita and his son. The Doctor initially refuses, but is convinced to stay. Before they depart, Jackson enquires after the Doctor's many companions, and the Doctor replies that in the end, they find something better than him. The pair then head off for a Christmas dinner in honour of those they have lost.
