The one where we get an exclusive "behind the scenes" look at what's coming soon...
Mission to the Unknown is a strange beast in that it's a side-step interlude in the ongoing narrative of Doctor Who. It went out under the banner of Doctor Who, and even credited William Hartnell, but none of the series regulars appeared in it. It featured just a small guest cast of humans and aliens, plus, of course, the Daleks!
In hindsight, it is a little odd that they plonked this prelude to The Daleks' Master Plan between Galaxy 4 and The Myth Makers, because after seeing this episode, viewers would fully expect the story to carry on, but they were instead taken to Ancient Greece for four weeks, where there were no Daleks and no threat to the solar system. It must have been a little confusing, as proven by the BBC's Audience Research Report on Temple of Secrets (episode 1 of The Myth Makers) which said: "Some viewers, having seen the previous week's Doctor Who in which the Daleks turned up again, were apparently unprepared for the switching of the scene back in time to the Trojan war (several wondered if a mistake had been made and the wrong programme put out) and were perplexed and rather resentful." One viewer wrote: "This programme has run too long and appears to have run out of ideas, episodes lately very poor and do not even tie up with the previous weeks."