The Key to Time: The Pirate Planet (Fourth Doctor).
Douglas Adams' first contribution to Doctor Who and it is very well done. The plot is interesting, a traveling planet that destroys and mines other planets by enveloping them and so are the characters. The standout of course is the Captain. Purchase's bellicose performance was perfect. His exclamations were always funny:
"By the curled fangs of the sky demon!"
I figured the nurse's role would get bigger once we found out who she was but I didn't expect her to be the primary antagonist, the holo-projection of Queen Xanxia. My only qualm was that the serial ended rather abruptly.
The Key to Time: The Stones of Blood (Fourth Doctor).
I always enjoy the use of silicon based lifeforms in science fiction shows. The fact that the Megara weren't men in suits made it better. It was fun seeing Professor Rumford out-eccentric the Doctor in several scenes. The look of pain on Tom Baker's face was fantastic when he realized he'd been beaten. I also liked how the serial transitioned from horror in episodes 1 and 2 to science fiction in 3 and 4. However the trial sequence in episode four slowed the serial down and seemed out of place with the rest of the story.