I quite like this one. The incredibly versatile Bernard Kay is clearly the most interesting character by a mile, although Ashe (a brilliant John Ringham who couldn't be further from the slimy Tlotoxl) and his daughter (a young Helen Worth AKA Gail from Coronation Street!) add a certain amount of colour. But the robot is dismal, and I think this one of Jo's very worst stories. She's absolutely hopeless. A trio of trios (the three species within the caves on Uxarieus; the three members of the I.M.C.; and the three groups themselves) help to focus the story, which I think is one of Malcolm Hulke's most political, and (not coincidentally) one of his best.

There's not many Pertwee stories I will re-visit again and again, and this is not one of those few. But it is nearer the top of the pile for me, and a highlight in what for me is a very shaky series.