I don't know, maybe the budget was running out or they wanted to film this quickly but this isn't really one of the better outings for the "Doctor" (Tom Baker) and his companions. It follows on, a little, from the previous "Ark in Space" series with them on Earth ensuring the passengers from that vessel are safely ensconced and safe. What they discover, though, is that what humans there are have been captured by a menacing "Sontaran" soldier called "Styre" (Kevin Lindsay - by now a bit of a regular in these leathers). It transpires that he has been sent by his high command to evaluate the strengths and weaknesses of human beings so he can feed that back to his bosses - poised to invade! It falls to the team to thwart the plan and, well, that's about the height of this disappointingly predicable and rather weakly structured story. There are only four episodes but the story could probably be adequately conveyed in two - set amidst a rocky outcrop largely devoid of much action save for everyone falling over/down and doing their own version of "Slip Sliding Away". It's watchable enough, just not one I think you're likely to remember.