The "Shada" story on Doctor Who is an exceptionally special Who tale. Actually, to be more precise, it isn't necessarily the story that is special, it is the story surrounding the story which makes it so important.
"Shada" was written by Douglas Adams and was meant to close out season 17 (1979-1980) of the original who series and features Tom Baker as the Doctor and Lalla Ward as his companion, Romana. However, due to production strikes, filming was never completed on "Shada." The story was eventually released to VHS in 1992 and features Baker reminiscing on the story and providing some voiceover to connect the filmed bits together (most of the original filming seems to have taken place). "Shada" has also been turned into an audio drama with the Eighth Doctor (Paul McGann) stepping in for the Fourth (an animated Flash version of this story is included as a bonus feature).
The January 2013 DVD release is the 1992 VHS version along with two discs worth of supplemental material including—outside of the Eighth Doctor "Shada"—the very good feature-length documentary, More than 30 Years in the TARDIS, which tells the tale of the series as a whole. There is also a bonus feature on other worker strikes that impacted the production of the original series. And more, but we'll discuss some of that later.
So, after that buildup, what is "Shada" all about?
The story features the Doctor and Romana going to then present-day England (1979), in order to have the Doctor visit a professor at Cambridge. The Doctor, it seems, has gotten a distress call from the Professor, who is named Chronotis and who is actually a retired Time Lord (played by Denis Carey). There is also, quite naturally, an evil alien present. This villain is a humanoid named Skagra (Christopher Neame) who has designs on conquering the universe and to do this, he needs Chronotis' knowledge of where the Time Lord prison planet, Shada, is located.
Wow, okay, so it doesn't sound like the greatest of stories, or rather it sounds eerily similar to any number of other Doctor Who stories, but the truth is that it is really quite good. Baker is in top form as the Doctor, and Ward is always fun as Romana. However, it is Carey who steals the show as Chronotis. Chronotis is a bumbling old Time Lord who can't quite remember what he is doing and why. The story has the sort of whimsy one generally attributes to Douglas Adams and Chronotis' appearance and bumbling but ridiculously smart nature is the exact same type of character that Adams fans will be familiar with in the form of Hitchhiker's Guide to the Galaxy's Slartibartfast (or with Dirk Gently's Chronotis, but that's cheating because he's more or less the same character as the one in "Shada").