London Event

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. (MA: Downtime)

The Event itself
The Event began with the presence of mist across London, causing a rash of disappearances; Central London was evacuated and the area cordoned off. Two days later, a web-like fungus infected the Underground, causing that to be closed off. Two days after that, the Yeti were sighted, and began cutting telephone lines.

The British Army, with a command hub based in a Second World War transit camp at Goodge Street station, carried out operations to try and hold back both fungus & Yeti. This lasted 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 Second Doctor and new commander Colonel Lethbridge-Stewart. The Doctor managed to sever the Intelligence's connection to Earth, but almost the entire Army platoon had been wiped out in the operation. (DW: The Web of Fear)

Aftermath
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 would later report it was more likely an attack by an undisclosed foreign power. (MA: Who Killed Kennedy)

The London Event directly led to the creation of UNIT. (MA: Who Killed Kennedy, DW: The Invasion)