The TARDIS narrowly avoids becoming engulfed in a cobwebby substance in space. It arrives in the London Underground railway system, the tunnels of which are being overrun by the web and by the Great Intelligence's robot Yeti. The time travellers learn 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.
Patrick Troughton
The Doctor
Frazer Hines
Jamie McCrimmon
Deborah Watling
Victoria Waterfield
Nicholas Courtney
Colonel Lethbridge-Stewart
Jack Watling
Professor Travers
Tina Packer
Anne Travers
Jon Rollason
Harold Chorley
Ralph Watson
Captain Knight
