Watched this again last night (first time I've seen it in colour!)
Some great scenes with the Master again. I really liked that his fear was the Doctor winning and gloating over him. The scene where one of the Doctor's hearts stops beating and the Master is deeply concerned and tries to save him is also very interesting and nice to see; I know he said he wanted the Doctor for the machine and for his own benefit, but I think the main reason he didn't want the Doctor dead was because he enjoys his rivalry. I liked that a lot.
I love all of the UNIT scenes, too. It really does feel like a family - a dysfunctional and arguing one, of course, but a family nonetheless. I really enjoy the Brig, Yates and Benton's characters and how they interact not only with the Doctor but with each other, like the scenes where the Brig loses his temper with Yates or calls Benton names.
A really good story. One of my favourites, and possibly my favourite Pertwee story so far! 5 out of 5 from me!