Zygon gambit

The Zygon gambit was the attempted invasion of Earth by the Zygons in the late 20th century, which was thwarted by the Fourth Doctor and UNIT. (TV: Terror of the Zygons, Remembrance of the Daleks, PROSE: The Dimension Riders)

Background
At some point, the Zygon homeworld of Zygor was destroyed in a stellar explosion. (PROSE: Doctor Who and the Loch Ness Monster) During the Zygon gambit, Broton said his world was destroyed in a "recent catastrophe", (TV: Terror of the Zygons) although his conception of time was unconnected to the human time-scale. (PROSE: Doctor Who and the Loch Ness Monster) According to one account, the planet was destroyed because of the Zygon war against the Xaranti. (PROSE: The Bodysnatchers) Most accounts agreed that the Zygons lost their world when it burned during the Last Great Time War, (TV: The Day of the Doctor, WC: Zygon Stats, PROSE: The Whoniverse) with Elizabeth I, who was posing as a Zygon in 1562, claiming it burned during the first days of the conflict. (TV: The Day of the Doctor)

Many Zygons were able to escape Zygor's destruction. They were forced to become nomadic refugees searching for a new planet. (PROSE: The Bodysnatchers) The Zygons who would enact the 20th century gambit had came to Earth in the 12th century but would not be the last group to arrive. (TV: Terror of the Zygons) Others arrived in the 16th century and plotted to invade in the 21st century, (TV: The Day of the Doctor) in 1909, (PROSE: Sting of the Zygons) and in the late-20th century. (HOMEVID: Zygon: When Being You Just Isn't Enough)

Aftermath
The invasion was remembered as taking place in the "seventies, eighties". UNIT naval surgeon Harry Sullivan worked at Porton Down on the captured Zygons, resulting in the development of a nerve gas designated Z-67, capable of unraveling Zygon DNA. (TV: The Zygon Invasion)

When Ace voiced skepticism towards the Shoreditch Incident of 1963 begin forgotten, the Seventh Doctor asked her if she remembered the Zygon gambit and the Loch Ness Monsters, correctly anticipating that she did not. (TV: Remembrance of the Daleks) In 2007, whilst competing with Rose Tyler, Sarah Jane successfully surprised her when she mentioned having encountered the Loch Ness Monster. (TV: School Reunion)