The one where the Doctor turns the Cybermats on their masters...
I've said it before, but I'll say it again - I love Harry Sullivan. How can you not love the bumbling old fool, played so perfectly by Ian Marter? At the start of this episode, he's basically responsible for nearly killing the Doctor twice over. "Harry, were you trying to undo this?" asks the Doctor of the explosive bomb buckle. "Well, naturally," Harry innocently replies. "Did you make the rocks fall, Harry?" adds the Doctor, to which Harry responds: "Er, well, I suppose I must have done, yes."
"HARRY SULLIVAN IS AN IMBECILE!" hollers the Doctor, creating one of the most memorable moments involving Tom Baker and Ian Marter. The following scene is wonderful too, all down to Marter's spot-on performance and characterisation. Harry's rambling summation of events to the Doctor is hilarious. The way he casually mentions Kellman's death as an aside, forgets what the Cybermen are called ("Terribly bad on names") and sums everything up with the beautifully understated: "Things have gone a bit wrong." It's Marter at full throttle, totally in control of his character, and that's why I love him.