Approximately 40 years after his Tibetan expedition in "The Abominable Snowmen", an elderly Professor Travers reactivates a control sphere during his studies. The sphere inserts into an intact robot Yeti from Tibet at a private collection in London and escapes. In the following days, London is beset by thick fog and a deadly web-like fungus begins to infest the London Underground. Professor Travers is brought to the Second World War deep-level shelter under Goodge Street tube station, where his daughter Anne has asked for his help to defeat the menace affecting the tube system. Also present are Captain Knight, the current leader of the military manning HQ, Staff-Sergeant Arnold, who acts as Knight's deputy and Harold Chorley, the only journalist allowed to report on the crisis.