The one where everything goes boom...
In keeping with the pace of the rest of the story, episode 6 is in no hurry to get anywhere fast. The first ten minutes is basically made up of people getting onto a rocket, people talking about not wanting to get onto a rocket, and the Doctor and the Master recreating a scene between the Doctor and Jo from episode 4. I think that's the problem with this story: there's potential in it, but it's all told at a leisurely, genial pace, with only occasional pockets of action to lift it out of talky melodrama into space adventure.
The Master's plan seems to be to take control of the long dormant doomsday weapon which the advanced people of Uxarieus developed generations ago, but never used properly. He aims to use this super-weapon to hold the galaxy to ransom (or the universe, he's vague on the details) so that he becomes supreme ruler. And the automatic response to this is voiced succinctly by our hero: what for?