The Doctor and Romana were on their way to Gallifrey when they slipped through a Charged Vacuum Emboitment which brings them to E-Space. E-Space seems to be, for all intents and purposes, pretty much identical to our universe except slightly greener. Look, it's Doctor Who, things like this are going to happen, best to just roll with it. Apparently the planet Alzarius occupies the same space-time co-ordinates in E-Space that Gallifrey occupies in the regular Whoniverse. Needless to say, the Doctor and Romana are surprised to see a forest when they were expected to be greeted by men in silly hats. Apparently, the TARDIS doesn't just use a simple camera to see what's outside. No, they have to use some technology that shows them the image at the co-ordinates outside the TARDIS. This technology can't handle the 'negative co-ordinates' in E-Space, so it shows them Gallifrey on the scanners.
While the Doctor and Romana are figuring all this out, on Alzarius we've got another allegory playing out. This one seems to be about blindly following authority and tradition. Oh, and watch out for that hubris, too. We've got a group of teens, including one named Adric. Adric wears a patch that is his Badge of Mathematical Excellence. Adric is a nerd. Adric's older brother leads this group of teens as a sort of 'fuck you mom and dad' to the previous generation who are mostly concerned with repairing their starship that crashed on this planet, oh so many generations ago. The starship, incidentally, is rather creatively named Starliner. Way to think outside the box there, guys. While harvesting fruits, the adults notice eggs appearing in them, something they hadn't seen for 50 years. This means Mistfall - a semi-regular occurrence which brings poisonous mist, spiders and humanoid creatures from the marshes which look like someone's attempt at a homemade Creature From The Black Lagoon costume who a) didn't want to get sued for copyright infringement and b) just didn't care that much. This little group is run by 3 people, called the Deciders. The Deciders decide that everyone must hide in the Starliner until Mistfall is over. See how that works?
Adric, in an attempt to prove he's not a nerd (he is) steals some of the fruits. He does this just as Mistfall begins and everyone else is making their way to the Starliner, as per what the Deciders decided. The senior Decider decides to go after Adric, falls in the swamp and delivers a rather cryptic message as his last words: "Tell Dexeter we've come full circle!" Dexeter is not, contrary to popular belief, a dubstep band, a cleaning product or an unholy mashup of Dexter from Showtime and Exeter from This Island Earth. He's the head scientist of the group and NOT one of the Deciders, in case you were keeping score at home. Once the Doctor decided to go out and investigate, things get interesting...
Quite a bit happens in this story. I may come across as kind of hard on it, but the plot is actually fairly involved, imaginative if a little derivative and keeps the viewer guessing pretty much right up until the end. The Doctor has a particularly great moment confronting the Deciders after Dexeter goes a piece too far in examining a captured Marshman. "NOT AN ALIBI, DECIDERS!" We get a temporarily possessed Romana, Adric stowing away on the TARDIS and no funny makeup for Tom Baker this time around. As a side note, the writer of this story was only 17!
One of the things I love about Who is the sheer scope of the universe, and these last 3 episodes are perfect examples. Each one features multiple different alien species (most with fairly elaborate backstories), different worlds with exotic environments (which sometimes have to be augmented by your imagination in these early episodes) and a wealth of ideas. No wonder the expanded Whoniverse has so much material - the audio plays from Big Finish, books, short stories, comics and I'm sure there's more than one fan film out there. I swear Who has been adapted to every storytelling medium save semaphore, Morse code and smoke signals. While the new Who may be a lot more slick, produced and effects-laden than classic Who, I feel that the new Who is doing a fine job in carrying on the tradition of expanding the universe.
I'm also a huge fan of MST3K, and the guys over at Rifftrax just released their riff of the non-canonical "Doctor Who and the Daleks". I'm sure I'll post a write-up soon.