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
as The Doctor
Frazer Hines
as Jamie McCrimmon
Deborah Watling
as Victoria Waterfield
Nicholas Courtney
as Colonel Lethbridge-Stewart
Jack Watling
as Professor Travers
Tina Packer
as Anne Travers
Jon Rollason
as Harold Chorley
Ralph Watson
as Captain Knight
Jack Woolgar
as Staff Sgt. Arnold
Richardson Morgan
as Corporal Blake
Rod Beacham
as Corporal Lane
Stephen Whittaker
as Craftsman Weams
