The Yeti knocks Travers unconscious and drags him out.
The Doctor, Jamie, Victoria and Evans search the tunnels for Chorley. Lethbridge-Stewart and Arnold find Knight, who confirms that the Yeti have cut off the route to the explosives. They devise an alternate route and set off.
The Doctor finds that the TARDIS is cut off by the fungus. The Doctor snips off a little piece for analysis. They return to the HQ to find a soldier encased in fungus and Travers missing. They revive Anne who tells them what happened. The Doctor wonders why they captured him instead of killing him. The fungus continues to advance, cutting them off. They wonder who is betraying them to the Great Intelligence and what it wants.
The Doctor explains about his TARDIS. Lethbride-Stewart is intrigued. He tells Knight he wants to rescue the TARDIS as a possible means of escape.
The Doctor hypothesizes that Travers was taken because he was close to a breakthrough. He and Anne return to the lab and examine the control sphere Travers had opened. Travers had built a device to take control of the sphere. They also realize that all the Yeti idols are missing.
Lethbridge-Stewart lays out a plan to approach Convent Garden and rescue the TARDIS.
The Doctor opens the tin of fungus to find it empty. He wonders if Evans has destroyed the sample. He asks Knight for permission to go to street level and retrieve the circuits he needs to finish Travers's control box.
Arnold finds the tunnel to Convent Garden filled with fungus. He and Lane don gas masks and wade into it with the trolley. Evans hears them scream. He pulls back the trolley to find it Lane dead and covered in fungus. Arnold is missing.
The Colonel encounters Yeti on the surface. They fight a desperate battle and are able to damage some of the creatures with explosives and a bazooka but lose all but two men. Yeti attack the Doctor and Knight at the electronics store. Knight is killed but the Yeti are recalled before the Doctor can be finished off. The Doctor finds a Yeti idol in Knight's pocket.
The Colonel and Blake retreat to a warehouse but the Yeti are still after them. Blake is killed and Lethbridge-Stewart escapes back to the HQ. As they discuss the situation, they hear the Yeti homing signal. The Colonel finds the Yeti model in one of his pockets. Before the Doctor can disable it, the door opens to reveal two Yeti and Professor Travers…