Many episodes from the First and Second Doctors (106, as I write this) are missing. As in, the tapes got straight-up thrown away or recorded over. A lot of myths have cropped up - such as the evil tape-trashing BBC employee or the tragic warehouse fire - but the truth is simple:
TV shows used to get wiped all the time.
It’s hard to imagine in this age of DVD sets and channels devoted to reruns, but back in the early years of television there wasn’t much to do with an episode once the broadcast was over. (Check this Wikipedia article for an overview of the practice and a heartbreakingly-long list of shows affected.)
For Doctor Who fans, this means that the 1963-1969 years are spottily available. Some serials are missing all their episodes, while many others are a patchwork of intact video and … not-intact video.
However! *cue upbeat Murray Gold music* Doctor Who, resilient little show it is, has an ace up its sleeve.
We still have ALL the audio.
A side effect of shows only being broadcast once is that some TV fans in The Olden Days would run audio recorders during an episode so they could listen to their favorite programs later. Thankfully, proto-Whovians were almost as ridiculously dedicated as they are today, because they recorded the audio for every single episode. Here in Futureville, those recordings have led to:
The BBC has put out CD sets of the audio from “missing” stories, complete with narration from the actors to explain whatever isn’t clear from the sound effects alone. They’re sort of like audio books on steroids. (Non-steroids’d audio readings of serial novelizations also exist.)
Fanmade pairings of audio with telesnaps (the weasel-primate ancestors to today’s screencaps.) Typically use captions to describe the action, though some just use the narrated audio.
Pretty much the pinnacle of awesome. The BBC has slowly been commissioning animation studios to fill in the gaps of partially-missing serials. They’re not Disney-style cartoon masterpieces, (can you imagine if they had that kind of budget???) but they get the job done with Flash-animated likenesses of the actors. So far The Invasion and The Reign of Terror have been produced, with The Ice Warriors recently announced for the animation treatment as well.
Deep down, every Whovian hopes the BBC will someday discover a secret stash of every episode and rain DVDs down upon the populace. (Which does occasionally happen - 2 episodes were found as recently as 2011.)
But if that doesn’t happen, we’ve got some pretty rad substitutes.