Original air dates April through May of 1964.
The Doctor, Susan, Barbara, and Ian wind up on Marinus, and are blackmailed into finding some “keys” needed to operate a planetary mind-control machine. Hijinks ensue.
The serials were all neatly done, and pretty self-contained, and pretty watchable. The Doctor took a more active role than he had been in some episodes, and was quite charming.
There were even some mind-controlling bug-eyed aliens.
Lots of mind control in the episode, and lots of places to think about what’s right and what’s not. Also an episode that could lead to a discussion of different systems of law. Pretty good science fiction, and actually meets the original goal to engage children and give them a chance for discussion, without being too heavy-handed.
The “good guy” who was trying to save the planet was just as much a mind-controlling bastard as any of the others. They all got their own.