While on a routine expedition to Omega 4, the Enterprise arrives to find another starship, the U.S.S. Exeter, in orbit circling the planet. Kirk and a party go to the ship and find it abandoned and everyone on board completely drained of water leaving behind only bodily minerals. They check the log and are warned not to return to their ship, so go to the surface of the planet and find Captain Tracey... who is now in charge of the local civilization.

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...