The one where General Carrington's plan comes unravelled...
There's a lot of exposition at the start of episode 7, with General Carrington explaining his motivation like a comic book super villain. The difference is that the villain here is written sympathetically, as a man who has been doing all of this for what he perceives is a good reason, his "moral duty". He is keen for the Doctor to understand his motivation, and why he's done what he's done. The Doctor certainly does understand ("You had to do what you had to do"), but of course he does not agree with it. Carrington actually seeks the Doctor's "approval" twice, at the top and tail of this episode, and John Abineri gives a nice performance. It's the old adage that the bad guys never think that what they're doing is evil; they think what they're doing is for a good reason. As a certain Osirian once said: "Your evil is my good."
It's also interesting to note that all of this plotting and killing was triggered by the accidental death of astronaut Jim Daniels on the Mars Probe 6 mission. Carrington lost a friend because the aliens didn't understand that their touch would kill. It's all one big misunderstanding through a lack of communication.