I'd forgotten just how gritty Doctor Who could be in the Pertwee era.
This was one of the Doctor/Master stories that I don't think I've ever seen, and I always enjoy the Master's convoluted plots (and the way he usually has to ask the Doctor for help when things go wrong).
Here we have a prison riot, with UNIT storming the prison and shooting several of the prisoners dead, after some of the prisoners ambushed a UNIT convoy and stole a missile for the Master. And the Doctor is tortured. To quote Sue from Adventures with the Wife in Space: "When does Doctor Who turn back into a children’s television show again? This is more like Straw Dogs."
It was good, earlier in the story, to see parts for two Chinese actors, actually speaking Hokkien, with sub-titles. This was the dialect that the actress playing the Chinese Captain spoke, so she became the language coach for Jon Pertwee - though she said that she couldn't really understand what he was trying to say when she listened to the dialogue again. Still, it was a fairly unusual thing to attempt in 1971.
And Jo was really quite a useful companion - at one point she even disarms one of the prisoners as she tries to escape.
I was very impressed with the prison, too, which turns out to be Dover Castle (they couldn't film at a real prison for fairly obvious reasons) - but they did get a real missile from the RAF!