#24: The Dagger of the Mind
In case you forgot: Kirk and a pretty lady doctor investigate a penal colony while Spock and McCoy try to get answers from an escaped assistant. Turns out the head doctor at the penal colony, Dr. Adams, has been using a brainwave altering device to manage and rehabilitate the inmates, though it's less a rehabilitation technique and more a method of torture.
Another episode with McCoy and Spock working together, all right! It's also interesting (and refreshing) to note that Spock doesn't immediately discount McCoy's intuition about Van Gelder's condition. Maybe Spock recognizes the value of a good hunch now and then, or maybe he realizes something is up as soon as the computer reveals that Van Gelder isn't an inmate but an assistant—that's Doctor Van Gelder to you. Speaking of Spock, this is the first episode we see with the Vulcan mind meld, so the episode would be noteworthy for that regardless of anything else.
Dr. Adams also manages to be a frightening menace. There's something about the antagonists who welcome Kirk in with smiles and open arms who are creepier and more threatening than the standard monster of the week/Klingon war ship/etc. The neural neutralizer is a fun little sci-fi plot device, and Adams dying at the hands of it is fitting comeuppance.
But a Trek episode wouldn't be a Trek episode with a poorly-written woman character! Dr. Helen Noel is, for the most part, bland and uninteresting. She has no personality aside from a schoolgirl crush on Kirk and a naive trust in Dr. Adams (does she not notice the prisoner's uniformly blank stares?). At least the episode gives her some smarts in the Enterprise versus Adams showdown; she's bright enough to feign unconsciousness (kicking a guy into live wires, gruesome!) and grab a weapon at the first available chance. But that's all she gets.
In general, the episode moves at an engaging clip until Kirk insists on trying what we know to be the neural neutralizer, at which point it starts to lag. That time could have been better spent on the rehabilitation of Dr. Van Gelder and Kirk. We don't really get to see how the work of the neural neutralizer is undone: is it the same machine, just in better hands? Is it a lot of therapy or hypnosis? Does it always work? Answers to those questions would have been more interesting than watching Kirk spaz out in a futuristic dentist's office.