The frustrating thing about this story is that it almost, almost works. There are so many good ideas that just need a tiny bit of tweaking on one level or another to turn this into one of the classic "base under siege" stories. The Sensorites work brilliantly as an enemy, conceptually; they're telepathic monsters that feed on your emotions to control you, picking off the crew one by one by turning the enthralled crew members into weapons to terrify the others into submission. That's brilliant, but the script never properly exploits the sense of terror and paranoia the idea evokes--John, the crew member whose mind has been shattered by fear and guilt, has some wonderful moments as he tries to resist harming the others, but it's only an isolated minute or two in a script that's otherwise fairly tension-light.
The script also needs a larger cast for that concept to work--they need to have paranoia, tension, something to work against the Doctor's attempts to calm them down and get them to work together. No only are there not enough people for that, but Lorne Cossette and Ilona Rogers as Maitland and Carol play their parts so flat that you don't buy them as being terrified into submission, or even controlled, or...okay, I'll admit it, I don't know what they were going for, but they failed at it. It may be the direction that's to blame, though...Pinfield doesn't get good performances out of the regulars, either. Hartnell comes off as pompous and belligerent, and Ian and Barbara wind up just sort of flapping around the edges of the plot. (Although the script doesn't give them much to do, either.) The only person who should feel happy this week is Carole Ann Ford, who finally gets something to do as the one person who can communicate with the Sensorites on their level.
And the Sensorites, well...look, from the neck up, they're great. They have these crenelated, elongated heads with weird, wispy hair that makes them look ancient and arrogant and wise and strange...
And then from the neck down, they're wearing footie pajamas. There's just no getting around it--there is no way to make any monster scary when they're wearing footie pajamas. Just try to imagine it in your head, the Weeping Angels or the Krillitanes in footie pajamas. Reduces the menace factor pretty heavily, doesn't it? Couple that with the fact that they're apparently scared of the dark and flinch at loud noises, and it not only kills most of the drama of the slow-motion "chase" through the spaceship (and it's about here that I feel the need to mention that the spaceship's design scheme looks a little too much like brickwork to be convincing) but it makes Susan's decision to let herself be taken hostage, which should be a dramatic game-changer, look foolish. "If I don't, they'll kill you all, Grandfather! That is, unless you make a loud noise or dim the lights a bit. Then they'll have to leg it with their ludicrous feet."
Ultimately, a big part of the problem is that the script is trying to introduce a number of concepts that its audience doesn't have the cultural vocabulary to understand yet. Later Doctor Who stories would be able to take it as read that the viewers would know what telepathy is and how it works, as well as being able to drop things like "molybdenum" into the scripts without a pause for a brief metallurgy lesson. So much of the script is spent just getting across concepts that later generations would immediately understood that they can't properly set up the real conflict, let alone the twist that the Sensorites are more desperate than evil. In that respect, it's almost unfair to judge this episode by modern standards, since most modern Doctor Who can pace itself faster and do more with the same ideas precisely because of stories like this doing the heavy lifting in terms of explaining the concepts to fans. Which means we're back to the same conclusion as last episode--everything done here will be done better later, but that's a mark of this story's influence as much as its flaws.