Finally, back in Episode Three, once The Doctor's specially requested lead lined room has been completed, is The Doctor, at last, ready to let his guard down and reveal his plan to Borusa. That the story has managed to keep The Doctor's ruse against the Vardans (and the viewers) for over two and a half episodes is quite remarkable.
Despite some dodgy aspects of the production, this story has managed to be entertaining as well as enigmatic. And any of those faults in the production can firmly be attributed to the industrial action that forced the production crew into a disused mental hospital for most of the shooting, be it on OB videotape or film. The only thing more hectic than the production schedule was the actual writing of the story, which was shared by script editor Anthony Read and producer Graham Williams. The fact that any of this serial managed to make it on to the screen is in itself an amazement.
There's also the first strains of Dudley Simpson's specially written theme for The Doctor in this episode. It would be used sporadically over the course of the next two seasons, and is one of the rare occasions in pre-1980s Who that feature a set theme for a character (the obvious exception being for that of The Master, who couldn't go three scenes in the Pertwee era without his specially written theme being heard.
The episode famously ends with the Sontarans appearing on the steps of the Panoptican, ready to invade Gallifrey, in one of the series' best ever cliffhangers. Unlike other six-part stories that have been loosely structured as a four-part story with a two-part prologue or epilogue, The Invasion of Time looked and felt like it was wrapping up for real at the end of Episode Four, thus making the sudden appearance of the clone warriors all that more shocking.