How has this happened? The team learn that the impending disaster was triggered nearly forty years before, after their previous encounter with the Intelligence. Professor Travers accidentally reactivated a Yeti, thus giving the Intelligence the ammunition it needed to plan another invasion attempt.
The aim is simple: the Intelligence wants to drain the Doctor's mind. Army forces, led by Colonel Lethbridge-Stewart, help the Doctor and his team race against time to stop them in their mission. Can they make it?