Jamie and Evans try to escape through an arch but find a Yeti carrying a glass pyramid. Jamie has Evans shoot the pyramid but there is no effect. They escape the fungus just in time.
Victoria wanders the tunnels looking for the Doctor. Suddenly both the Doctor and another soldier, Colonel Lethbridge-Stewart, appear. After hearing the Doctor and Victoria discuss Travers, Stewart has them all go back to HQ. Knight and Arnold wonder where the colonel has come from, as does the Doctor, who was simply knocked out by the earlier explosion.
Lethbridge-Stewart tells Knight that he is taking over the operation and presents his credentials. He was with the ammunition party and wandered the tunnels until he found the Doctor. Travers eagerly greets the Doctor and brings him to the lab. They wonder why the Great Intelligence brought the Doctor there.
The reporter Chorley is growing hysterical and insists on finding a way out. When he hears about a door to the surface, he bolts.
Travers admits to the Doctor that he brought back artefacts from Tibet. He fiddled with an intact control sphere until one day it just disappeared and reactivated the Yeti.
Evans decides to get out of the tunnels while he still can, leaving Jamie behind.
Lethbridge-Stewart holds a briefing. He explains that a strange mist began to appear and people vanished into it. After London was evacuated, a fungus appeared in the underground followed by the Yeti. The mist is expanding and they can do nothing to stop it. The Yeti have torn out the phone lines, cutting them off. Chorley returns, still determined to find the way out.
Evans returns to Jamie and admits that he couldn't get out.
The fungus is advancing and Travers finds they may only have hours left. The Doctor suggests blowing a couple of tunnels to slow down the fungus. The Doctor suggests setting the explosives on a trolley and blowing it up while it's moving to prevent the Yeti from cocooning it.
Victoria finds a little Yeti statue that was used to guide the Yeti in Tibet. One of them is missing. Someone has set it up to draw the Yeti to the explosives store. Blake finds the smashed padlock and Yeti statue and reports it to the Doctor. He investigates to find the explosives room filled with fungus.
Lethbridge-Stewart decides to go to Holborn to pick up the other set of explosives. The Doctor points out that someone has betrayed them and is controlling the Yeti. When Chorley hears about the TARDIS and the plan to destroy the tunnel, he locks them in the room and races out to stop the soldiers, passing Jamie and Evans as they return.
Travers is working when he hears a horrible scream. He investigates to find Weams dead and Yeti model next to him. He looks up to see the menacing form of a Yeti…