This is a Doctor Who
story that never was. The story concerns a old teacher of the Doctor's from Gallifrey
who is now a professor at Oxford. It was a fourth Doctor story (played by Tom Baker
) and his assistant at the time was the post-"Key to Time" Romana
(played by Lalla Ward
I believe that the Shada of the Title is a Gallifreyan Prison Planet.
Shada was never completed due to a strike of studio technicians that caused the cancellation of the final studio filming session.
It should have been the last story of season 17 and would have made a much stronger season finale than "Horns of the Nimon". Although it would not have taken a lot of work to complete the story it was not finished. The rumour is that the new producer (John Nathan-Turner who produced all the remaining series on the BBC) did not want to start his run on the program with someone else's story.
The script for this story was written by Douglas Adams (of Hitch-Hiker's Guide to the Galaxy fame). He recycled a lot of the ideas for his novel Dirk Gently's Holistic Detective Agency.
There is a VHS tape of the story available with voice over etc. in the place of the missing material (a lot of the effects shots were never done either).
Footage from the story was used in the story "The Five Doctors" when Tom Baker could not appear himself.