The TARDIS narrowly escapes being engulfed in a cobwebby substance in space. It arrives in the London Underground, the tunnels of which are being overrun by the web and the Great Intelligence’s robotic Yeti.

The time travellers lean that this crisis was precipitated when Professor Travers, whom they first met in the Himalayas, some thirty years earlier, accidentally caused one of the Yeti to be reactivated, opening the way for the Intelligence to invade again.

The travellers work alongside army forces – led initially by Captain Knight, and then by Colonel Lethbridge-Stewart – as they battle the alien menace, hampered by the fact that one of their number has fallen under the Intelligence’s influence and is a traitor in their midst.

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