Second Doctor, Fourth Serial, 33rd Story.
Summary: Following a disastrous attempt to control the TARDIS, we find ourselves on the Moon. After a few missteps in the low gravity, they encounter a Moonbase, a weather center (They control the weather on the Earth) and, unfortunately, illness. Viral infection is the least of their problems, as there's a spy at work on the base, trying to take it over. Bodies are going missing from sick bay, the Graviton beam is malfunctioing and a Hurricane is threatening the USA, and the station crew can't seem to figure out what's going on! Worse, the disease and everything else is an invention of the spies - the Cybermen!
The Doctor: I would never have expected the Doctor to not want to explore, but he seems awfully reluctant to explore the moon. His knowledge of time comes in handy, mostly guessing nearly right! He's clever, though, figuring out quickly that the virus is artificial. Once bodies start vanishing, the Doctor is eager enough to participate - to fight Evil. You can hear the capital E when he says it! I think Troughton's finally getting the role, and his performance here is wonderful.
Rating: Enjoy it. It's not the best, but it's much stronger than Underwater Menace was. As much as I dislike the characters being slidelined for long periods, three companions is a bit much, and keeping Jamie (and Ben) out of the way worked to move things along. The second Episode is really slow, however, and is barely helped by the strong performances of the Doctor and Polly. Otherwise it's a pretty strong story, with a good villain. There's a lot of threats, but it doesn't feel like a pile O'Threats made just to complicate things, which is rare.