I haven't been writing all that much of late, partially because of RL (real life) responsibilities, but also because I've been spending a lot of free time watching old Doctor Who episodes. My mom was the real Doctor Who fan and has most of the episodes that aired on PBS here in the states on a zillion video tapes, but after I watched the last four seasons of the reprised Doctor Who I've gotten more interested in seeing the old episodes again. Growing up, most of the kids my age were familiar with only one of the Doctors (Tom Baker), and to them he was and is the one, the only, the quintessential Doctor (the genuine article!) - but in my house it was all about Patrick Troughton and Jon Pertwee.
Sometime in the '70s (and before PBS was airing them in the US), the BBC began taping over the older episodes eventually erasing 108 episodes of William Hartnell (the first Doctor) and Pat Troughton. Doctor Who was (and to some extent still is) a kid's show, and the BBC was pumping them out. I guess they figured the show, which was produced on a shoe string budget, many times with actors obviously reading their lines off cue cards, and notoriously cheezy special effects - wasn't going to have a long shelf life. Boy, were they wrong. The show already had die hard fans back then (mid sixties) who would set up their personal reel to reel tape recorders to record the audio off the television broadcasts so they could listen to the shows again later (there were no commercially available video recorders at that time). These guys had no idea that their recordings would one day become the only extant audio source material for the many missing shows.
Eventually as fans found out the horrible truth - that there were over a hundred episodes they would never be able to see again, they set about organizing to reconstruct them. Gathering surviving clips (rare), telesnaps (poor quality pictures taken directly off a television monitor for reference purposes), publicity stills (photos) taken on the set at the time, and using reconstructed photos (custom images with an actor's likeness where no other images exist), they merged these together with the fan-salvaged audio and appropriate title and credits to re-create the episodes in what appears a lot like a slide-show version of a TV show. The quality of the images and audio is poor, and action scenes while sometimes explained through use of captions, are largely a mystery and left to the imagination. Other fans have worked to find and post online the complete scripts for the original shows with stage direction and filming notes so if you have the script available you can read along and get a good sense for what's happening. Watching the reconstructed episodes is a lot more work than kicking back and watching the boob tube, but many Doctor Who fans are willing, and especially if they are diehard Hartnell- or Troughton-as-Doctor fans.
The first reconstruction I saw was at I-Con two years ago where The Macra Terror was shown. I had no idea what the hell was going on, but it was also strangely compelling. After talking to my mom about it, she was very keen to see them as well and since their internet connection is a little slow I set about to grab copies for her.
The reconstructions are out there for fans to find. For a long time if you had the gumption you could send a VHS tape to the folks that made them and their army of loyal fan copiers and they'd shoot you back a copy. In this day and age you kind of expect to be able to find things like this online, and while I will link to some resources at the end of this post I am not going to link to my favorite sources for the final videos online at this time. As usual, it always comes down to what you think the BBC lawyers will let you get away with. That said, its very important for fans to support the BBC releases as they come out so that they continue to work on releasing more!
When the BBC finished up The Invasion a couple years ago I hoped it would be the beginning of a wave of BBC funded reconstructions based on the fan work already done. The Invasion is an eight episode Pat Troughton story where episodes 1 and 4 had been lost. The BBC had released the surviving episodes of The Invasion on video in the 90s with some summary narrations for the missing episodes, but the 2007 DVD release of The Invasion [amazon] included a full-up flash animation reconstruction of episodes 1 and 4! The animation fit very well with the rest of the episode which was in black and white, and if anything added a surreality to the story which made me wonder why they couldn't do all the missing episodes using this method. The animation was done by the same chaps who did the completely animated Dr Who episode entitled "The Scream of the Shalka" which was released on the BBC website in 2003. I think the answer to this quesion is, as usual, money - but man, it would be cool.
Here are some Doctor Who related links which I found were the most useful:
Doctor Who wiki http://tardis.wikia.com/wiki/Doctor_Who_Wiki
Loose Cannon Doctor Who Reconstructions http://www.recons.com/
Doctor Who Scripts Project http://homepages.bw.edu/~jcurtis/Scripts/scripts_project.htm