An Enterprise landing party beams down to investigate a distress call. Two humans appear and thank Kirk for their speedy response to the distress call. They then demand that Kirk surrender the Enterprise and paralyze the party before they can stop them. The survivors are from far away and need the Enterprise to reach their home.

The TARDIS is caught by a mysterious force which causes it to materialise in present-day London. Forty years have passed since their adventure in Tibet, and their old friend Professor Travers has succeeded in reactivating one of the dormant Yeti control spheres - only to summon the Great Intelligence back to Earth. The streets of London are choked with fog, Yeti roam the Underground, and a beleaguered team of soldiers and scientists struggles to find a solution before the city falls completely to the invader. A new commander arrives, Colonel Lethbridge-Stewart - but is he really the new commander of the garrison or is he the Intelligence's new human agent? And if he isn't, then who is? The Doctor must find the traitor before the base is overrun by the Web and London falls...