This one's easy for me just because I love these types of plots. Would've even liked a longer serial for this one! The whole mystery around the situation ending up being such a simple answer feels perfect to me. Something absurd and at the same time so interesting, just what this show is. I'm weak for bottle episodes that force characters to interact and carry the whole thing. The Doctor's dynamic with, well, everyone is marvelous. His personality clash with Ian? The way he ends up being so fond of Barbara? And, in general, all of his love for Susan... Simple two-parter that actually shows a lot of things about them <3