The best thing about this one is Delgado as the Master. Accept no substitutes! He is the real thing! And the Brigadier is fun as well.
Interesting that the Doctor spends the first episode whining that the military want to blow the Axons out of the sky, whereas in fact as it transpires this was the right idea (if unimplementable).
Jo is useless as usual, and gets put in a position of serious danger by ignoring a direct instruction. The only interesting thing to say about her is that she has perhaps her first semi-romantic moment with the visiting American. (What the American is doing there is not at all clear.)
But in general I rather liked it. I've been a bit sceptical of Bob Baker and Dave Martin's scripts (deeply unimpressed by The Mutants, The Three Doctors and The Sontaran Experiment), but this was a good 'un; directed by Michael Ferguson who also did the rather good Seeds of Death and the underrated War Machines.