Peculiar story. Dr Who, Vivki, Steven, and the TARDIS don't appear at all in this story, The primary characters seem to be a bunch of human Space Cadet types who have crash landed on a planet where a bunch of alien Space Cadets have also crash landed, but only one ship can leave. The Daleks are also involved although they don't make much of an appearence either. "Mission to the Unknown" is a single episode and there's no continuity between this story and the previous Dr Who story, "Galaxy 4", or to the next story, "The Myth Makers" although there is a link to the "The Dalek's Master Plan" which was 5 weeks later.
One theory of why this happened was that the BBC was trying to create a Dr Who spinoff without...Dr Who, but with the Daleks. They didn't bother to warn their audience that the show they were watching wasn't really a Dr Who show and that they were actually a test audience for a new series.
No copy of this episode remains. Was it copied over for another show, or did the BBC realize that it had done something stupid and deleted it?