The London Event was the later official name for the takeover of London and its underground rail system by robot Yeti controlled by the Great Intelligence. (PROSE: Downtime) There were multiple accounts as to when it happened. It either took place in August 1966, (PROSE: Who Killed Kennedy) 1968, (PROSE: Downtime, Revolution Man, HOMEVID: Global Conspiracy?) February 1969, (PROSE: The Forgotten Son) or March 1975. (PROSE: The Enfolded Time)
The event began with mist across central London, causing a rash of disappearances. On the fifth of the month (which may have been the start date), the mist had reached the Natural History Museum and the affected area was evacuated and cordoned off. The mist kept spreading, reaching South Kensington tube the next day; by the seventh, a web-like fungus infected the Underground, causing that to be closed off.
The British Army, from a command hub based in a Second World War transit camp at Goodge Street tube station, tried to hold back fungus and Yeti. Radios could not broadcast through the mist and while telephones were used at first, the Yeti soon destroyed the phone lines. The army tried laying their own lines between station bases. What the Yeti were was unclear to the average soldier, with Corporal Blake believing this attack must be a foreign power (and Travers was "off his nut" about aliens) while even weeks in, Weams was unaware the Yeti were robots.
At some point, the fungus ceased growing. Three weeks later, at which point the army believed the mist covered "the entire area enclosed by the Circle Line" the Intelligence brought the Second Doctor's TARDIS to London after ensnaring it in space. (TV: The Web of Fear) Ramón Salamander was caught in the temporal wake of the TARDIS and brought to London as well. (COMIC: The Heralds of Destruction)
The arrival of the Doctor also saw the fungus starting to grow faster than before, an engagement of troops at Holborn, a sustained attempt by the Intelligence to cut off the transit camp, and the arrival of the new commander Colonel Lethbridge-Stewart.
An army operation followed with the intent to retrieve the Second Doctor's TARDIS from the web fungus in the London Underground. Colonel Lethbridge-Stewart and seven other soldiers were ambushed by Yeti at Covent Garden as they attempted to reach the TARDIS from the surface. Although they managed to destroy a number of the robots, the soldiers were vastly outnumbered and soon ran out of ammunition. Only Colonel Lethbridge-Stewart, having lost all seven of his men, including Corporal Blake, made it back to HQ following the engagement. (TV: The Web of Fear)
The cover story for the event was that there had been a major nerve gas leak. While the government claimed this had been an industrial accident, journalist James Stevens later reported it was more likely an attack by an undisclosed foreign power. (PROSE: Who Killed Kennedy)
The London Event directly led to the creation of UNIT, (PROSE: Who Killed Kennedy, TV: The Invasion) after a meeting was held between Lethbridge-Stewart and Air Vice-Marshall Ian Gilmore. (PROSE: The Scales of Injustice, The Dogs of War)
A battered cardboard file containing a full Ministry of Defence report on the London Event was discovered by Sergeant Beagles in the security vault at UNIT H.Q. Being pre-UNIT, the report was not known to exist except on hard copy. Brigadier Crichton had hoped the file would be of help during the Great Intelligence's fourth attempted invasion of Earth in 1995. However, before the information contained therein could be acted upon, the file was stolen by Captain Cavendish. (PROSE: Downtime)