Doctor Who and the Web of Fear — Terrance Dicks

Forty years the Yeti had been quiet--Collector's item in a museum. Then without warning it awoke--and savagely murdered.

At about the same time patches of mist began to appear in Central London. People who lingered anytime in the mist were found dead, their faces smothered in cobwebs. The cobweb seeped down, penetrating the Underground System. Slowly it spread...

Then the Yeti reappeared, not just one but hordes, roaming the misty streets and cobwebbed tunnels, killing everyone in their path. Central London was gripped tight in a Web of Fear...