I can't actually call this the famous lost episode of the original Doctor Who namely because there are actually a number of episodes that have been officially lost. While some have resurfaced over the years since the BBC went about destroying these episodes, there are still quite a number of them from the first and second Doctors that are missing and will probably never be found. This particular episode, though, was only half made and never completed due to a strike at the BBC. It was also the last Doctor Who serial that Douglas Adams ever penned (as far as I am aware that is).
I don't remember too much of this serial, but the script came in a boxed set that contained a partially reconstructed version of the episode. Namely the recorded sections of the serial were edited and the sections of the serial that were not recorded at the time were placed in by way of subtitles over a frozen image. This was actually interesting to watch because we get to see how a television show is actually put together. In fact movies and series are never actually filmed as they appear, but rather in a sort of more haphazard way. Generally studio shots are done first and then the location shots are done elsewhere. Also, scenes in the same area are all filmed together, so when we actually watch this serial we discover that the end of the serial was filmed before much of the middle section.
It is a shame that some of the old Doctor Who episodes are missing and will never be recovered, particularly since I tend to prefer the original series than the new series. However, Doctor Who has never really died, and when the show was taken off the air in the late eighties, a series of 'New Adventures' (which have been a subject of a number of commentaries that I have written) were written and pretty much bridged the gap between the old and the new series. However the Doctor Who novels (which took a much more adult tone than the TV series) are not considered cannon.
The one thing that I can say about Doctor Who is that some of the earlier episodes, particularly the Tom Baker episodes, are actually quite scary. It is interesting that the Australian censors went at some of the episodes due to their horror content, and two particular serials, The Brain of Mobius, and The Deadly Assassin, were not shown on television due to the horror content. However, when Doctor Who shifted to a 2:00 am timeslot, they were eventually shown (and I believe that at that time they went through all of the available episodes, from the beginning to the end).