Terror of the Zygons was produced as part of the production schedule for Season 12, but was held for transmission from the end of that season to the beginning of Season 13, to coincide with a move from the new season starting at the beginning of the calendar year, to starting in late summer. Terror of the Zygons was also the last appearance of the Brigadier until Mawdryn Undead in Season 20. The season took a transmission break of two weeks over the Christmas 1975 period, between the broadcasts of The Android Invasion and The Brain of Morbius.
The Fourth Doctor is summoned to Earth by the Brigadier who is in Scotland investigating the mysterious loss of oil rigs, and discovers an alien invasion.
An expedition is murdered. The Doctor and Sarah Jane land on the planet at the same time as the expedition's rescue team, and are immediately taken prisoner as the suspected murderers.
The Doctor and Sarah Jane land in a duplicate of an English village, complete with androids created by aliens as a testing ground for their invasion plot.
Years ago, the renegade Time Lord known as Morbius was executed. When the Doctor and Sarah Jane arrive on the planet Karn, they discover that Dr Solon is creating a body so that Morbius may live again.
Scientists uncover two giant seedpods buried in the Antarctic permafrost for thousands of years. Ruthless plant-lover Harrison Chase decides he must have them for his rare collection, but the pods may spell the end of all non-plant life on Earth.
Broadcast
The entire season was broadcast from 30 August 1975 to 6 March 1976.
^Advanced release in region 2 as part of 'The Fourth Doctor Time Capsule' set with no special features on 29 July 2013
^The Region 2 release of Terror of the Zygons was released in advance as part of The Fourth Doctor Time Capsule box set without special features, and was later re-released on its own with special features.
^Also available as part of The Doctors Revisited 1-4 in Regions 1 and 4
^Only available as part of the U.N.I.T. Files box set in Regions 2 and 4. Only available individually in Region 1.