#23: Whom the Gods Destroy
In case you forgot: The Enterprise is delivering a medicine that will basically cure every mental illness ever to a penal colony, only to discover that the inmates are running the asylum. (So just one chemical, medical cure can be effective on a variety of alien races and undo all kinds of past traumas and crimes? I guess it was a more naive time.) The leader of the lunatics, Garth, can change form at will, but Spock outsmarts him and saves the day.
This episode never really drags or lulls, and Kirk and Spock do the best they can in a crazy situation. Both are clearly on their game in this one: Kirk has given the rare order for a "call sign" before beaming aboard (guess all those evil/possessed Kirks in the first two seasons finally convinced Kirk and the Enterprise to tighten their security) and Spock is able to rationally and logically deduce which of the Kirks is the real Jim.
Marta and Garth, leaders of the inmates, are also unhinged in a way that isn't cringe-inducing; it's actually pretty good entertainment. Garth, in particular, does a lot with a character that could just as easily be a cackling rip-off of the Joker. It's nice to see him played mostly straight, because that makes his madness that much more menacing. The banter between him and Marta is legitimately pretty funny; the two have decent rapport, if not chemistry.
Garth's shape-shifting power, though, is a little too much to swallow. The episode waves it away as "it's a technique he learned" but that seems pretty cheap. Would it be so much to give him a little hand-held remote or some other piece of alien technology?