The Invasion was to have been a sequel to a story in the previous season, The Web of Fear, and it was also intended to feature the return of two characters from that story, Professor Travers and his daughter Anne. As these actors were unavailable this lead to the substitution of two similar characters, Professor Watkins and his niece, Isobel. Further changes resulted in replacing the Yeti used in "Web" with the Cybermen, and expanding the story to eight episodes. Invasion was the fourth occasion Patrick Troughton's Doctor encountered the Cybermen, more than any other adversary.

Assisting the Cybermen was the unscrupulous Tobias Vaughn, played by Kevin Stoney. Stoney's previous Doctor Who role was arch-villain Mavic Chen in season three's The Daleks' Masterplan, where he found out the hard way that the Daleks do not share power with anyone. Perhaps the Cybermen will have more respect for their allies...

Brigadier Lethbridge Stewart, also previously seen in The Web of Fear, was back once more. The Invasion, significantly, was as the first of the "UNIT stories", and it served as a model for the largely Earth-bound Pertwee era that was to follow.

The Invasion follows on from the end of the previous story, The Mind Robber. The Doctor, Jamie and Zoe were left clinging to the console, but the Tardis reforms around them and they eventually find themselves on Earth in the present. The Doctor decides to look up his old friend Professor Travers, but he has let his place to Watkins, only for Watkins to go missing himself. The trail leads the Doctor to the giant corporation International Electromatics, run by the evil Vaughn. Despite assurances to the contrary, the Doctor is convinced that Vaughn knows a lot more than he is telling....

Episodes 1 and 4 of The Invasion are currently missing, although the remaining six episodes are widely available as an official BBC video release. Confusingly labelled "complete at last", despite the absence of episodes 1 and 4, "Brigadier" Nicholas Courtney provides short links in place of the missing episodes.

Introduction by GARY ZIMMER (ZIMMER@bandicoot.me.rmit.edu.au).