The first thing that struck me about this episode is the level of violence; rioting, punch-ups and shoot-outs and a rather high body count. All of which make it rather exciting, but not really suitable for children. The threat in this episode is also rather abstract and more likely to scare adults than kids. If you believe that Doctor Who is just a children's show then I'm sure this story will change your mind.
The main enemy is the Keller Machine, a intelligent mind-controlling machine, created by the Master, which develops a life of its own. The action takes place in a prison, where the machine is being used to remove criminal impulses from the inmates, but ends up resulting in a violent uprising. Some viewers don't like the Keller machine because it is too passive an enemy, but for me that is the very quality that makes it, and the whole concept of mind control, so unnerving. You can't fight it, nor can you hide from it. The subject of brainwashing was acute back in the days of the Cold War but is still relevant today, in the context of fundamentalism for example. The wider story revolves around a peace conference and negotiations with the Chinese, (which the Master hopes to disrupt). These are all strangely contemporary themes, so the story doesn't feel like it's based on old concepts.
The story is a good example of Doctor Who working at it's best, i.e within its limits. A great script from Don Houghton. No dodgy sets or monster costumes. The use of real props and locations.
The performances are great, though I will give special mention to Jon Pertwee, I can't help it. His distress during the Keller Machine attack and after as he come to terms with a very near death experience is a great piece of acting. Jo is manhandled in the most unchivalrous manner during the prison riot, spending much of the time with a gun pointed three inches from her face, but eventually gets her own back by shooting one of her assailants in the backside.
The machine scares its victims to death by confronting them with their greatest fear. The best moment for me is when the machine attacks the Master and we find out what he fears most in the universe, its quite a revelation.