Evans and Arnold follow the others into the tunnels to warn them that HQ has fallen. Evans, however, tries to run away.
The Doctor, not wanting whoever has betrayed them to know they have a Yeti under their control, leaves the Yeti behind and tells it to obey the Intelligence until otherwise instructed. They re-unite with the Colonel and Jamie, who report Arnold's survival. They then find Arnold, who tells them about the fall of the HQ. They are then surrounded by Yeti. Evans is also captured. Lethbridge-Stewart fakes an injury to allow Arnold to escape.
Victoria and Travers are brought into a room with a massive glass pyramid. They see someone in the shadows but can't determine who it is. The Professor blames himself for the troubles.
The Doctor tells Jamie about the reprogrammed Yeti. He's not sure which one it is but gives the control box to Jamie.
Arnold runs into Chorley, who has apparently been wandering helplessly through the tunnels. He says he found his way to Piccadilly, where he saw the machine, Victoria and Travers. Arnold wonders why the Yeti didn't kill Chorley but the reporter doesn't know.
Evans is brought to Piccadilly. The Doctor is led away and advises the others not to struggle when the Yeti come for them. Jamie hides to avoid being taken away. A Yeti brings a headset towards the Doctor, who uses the control box to deactivate the Yeti. He rewires the headset, puts it on and reactivates the Yeti. The other are brought to the room with the pyramid.
Jamie tries calling the Doctor's Yeti. One shows up but does not obey his commands. It turns on him with a roar.
The Doctor is brought in. Chorley shows up as does Arnold, who is under the control of the intelligence. Jamie is also brought in. A Yeti puts its arms around his neck and Arnold says he will be killed if the Doctor does not cooperate. The Doctor obediently steps into the machine.
Jamie orders the controlled Yeti to attack the other Yeti. In the chaos, they drag the Doctor out of the pyramid over his protests and rip the cords out, killing Arnold and disabling all of the Yeti. The Doctor, however, is outraged. He had reprogrammed the machine so that activating it would destroy the Great Intelligence. All they have done is cut off its contact with Earth.
The others, however, congratulate the Doctor for defeating the Great Intelligence. Chorley wants to make him into a national hero, but the Doctor decides to immediately leave in the TARDIS. He, Jamie and Victoria go into the tunnels and try to find the TARDIS before the trains come back…