Travers speaks with the voice of the Intelligence. It claims to have set a trap for the Doctor. It wants his mind so that it can drain it of thought and experience. It threatens to use his companions if he won't give in. If the Doctor gives in, the others can go free. It confirms that Travers wasn't working for it, but refuses to divulge who is. It takes Victoria and gives the Doctor 20 minutes to surrender. Evans wants to give the Doctor over to save the rest of them.
Jamie wants to go the surface and cut off the Yeti as they leave. Evans and Lethbridge-Stewart are initially opposed, because they don't know how to fight the Yeti. Eventually, the Colonel goes with Jamie while Evans stays behind. But they find their way blocked by the fungus.
The Doctor and Anne work on the control box. The Doctor repairs the Yeti sphere and gets it working.
When Jamie and the Colonel return, Evans hold a gun on them thinking one of them is the traitor. Lethbridge-Stewart ignores him and goes out into the tunnels with Jamie to track Victoria.
Victoria is brought to a station and hears the voice of the Intelligence. It releases Travers from its control. He tries to leave but a Yeti blocks the way.
The Doctor gets the control box working but only at short range. He now wants to reprogram the sphere to respond to verbal commands.
Travers sees Arnold in the shadows. He advises him to return to HQ and let the Doctor know where they are.
The Doctor tests out the sphere using voice control. If they can get it inside a Yeti, they'll control it.
Arnold finds Jamie and Lethbridge-Stewart. He explains that after he took the trolley into the fungus, he blacked out. Later he woke up and found himself wandering in the tunnels. Arnold tells them about Victoria and Travers and they all head back to HQ. Evens tells them about the Doctor and Anne going after a Yeti. Jamie and the Colonel head off, leaving Evans and Arnold behind.
The Doctor and Anne find their way blocked by fungus. Suddenly a Yeti appears behind them. The Doctor activates the control box and the Yeti stops. The Doctor replaces its control sphere with the altered one. They head out with their newly-controlled Yeti.
Evans patches up Arnold. He suddenly cries out as the fungus bursts through a wall and begin to fill the HQ…