Although a couple of episodes form this serial are missing it is still a pretty enjoyable one. It follows a theme that the previous serial took – humans on one side, aliens on the other – but it does have more interesting characters than the Cybermen in the Ice Warriors. Unfortunately the missing episodes serve as the introduction to them and when I picked it up a few episodes later it was not as clear as I would have liked it to be, although they still have engaging motives for their actions. Their design is nice apart from being a bit chunky and unwieldy as costumes (witness several of them boshing their heads on the low door frame) an their low hissing voices are sinister even if they are hard to hear all the time.
The plot uses them pretty well although the conclusion centering around the computer is a bit all over the place and I'm not entirely sure it worked. It is a little slow at times and I don't think it had the urgency and sense of danger that the Tomb of the Cybermen did better. The human cast are good and the standard of performances is generally better in this season than in previous ones (in terms of the supporting cast). Troughton gives another very good turn and even Hines is growing on me now that I have had a few chances to actually see him in a story or two; Watling is yet to really make an impression on me though – not all her fault, she really does feel like another in a long line of "passing through" companions who come and go is lost episodes.
All told though, an enjoyable enough serial with good characters in the aliens and humans, although the pace is a little slow and perhaps could have done with a bit more urgency and pace.