Mission to the Unknown is an unusual episode. It’s a single 25-minute tale, and thus the shortest regularly broadcast Doctor Who story. Moreover, it does not feature any of the regular cast, functioning instead as a prolonged ‘cold open’ for the next-but-one serial. For this reason, it’s often either skipped or dealt with briefly in written accounts of the Hartnell stories. For the purposes of this blog, however, The Daleks' Master Plan has plenty to cover as it is, leaving this post free to look at Kembel.
Where & When
The story is explicitly set in the year 4000, on the planet Kembel. This is an unclaimed world not directly controlled or claimed by any external power. The date is notably later than that of any of the earlier Hartnell serials, and The Daleks' Master Plan refers to the fact that technology has advanced since the time of the latter half of The Chase. Later stories will place it as (probably) happening during the early years of the Second Great and Bountiful Human Empire, which reaches its height a little over a century later.