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 are 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 London, causing a rash of disappearances; central London was evacuated and cordoned off. Two days later, a web-like fungus infected the Underground, causing that to be closed off. Two days after that, the Yeti began cutting telephone lines.
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. This continued for three weeks before a massacre of troops at Holborn, a sustained attempt by the Intelligence to cut off the transit camp and the arrival of the the Second Doctor and new commander Colonel Lethbridge-Stewart. The Doctor severed the Intelligence's connection to Earth, but almost the entire Army platoon had been wiped out in the operation. (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)
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)