#10: The Conscience of a King
In case you forgot: Kirk takes aboard a traveling theater company, because he suspects one of the actors of being Kodos the Executioner.
I freaking love Shakespeare. More than that, I freaking love Hamlet, and no episode is more Shakespeare, or more Hamlet, than "The Conscience of a King." More than references, though, the ethics of this episode—the circumstantial aspects of heroism, the subjectivity of history, the moral limits of vengeance, protecting loved ones—are pretty serious.
The actor in question is, obviously, Kodos the Executioner. We get multiple accounts of what happened on Tarsus IV, but none that are really satisfactory. The hard facts say that Kodos declared martial law in a state of emergency and that 4,000 people were shot outright to reduce demand on a devastated food supply. Spock says that Kodos used the opportunity as his own private eugenics experiment, but when pressed Kodos never goes on a crazy eugenics rant, only spouting some vague platitudes about a "revolution" and saying he had picked the people he thought to be the hardiest and most likely to survive. Of course it's not the most ethical move in the book, but not necessarily eugenics wackiness, either. Was Spock's interpretation over-reaching? Has Kodos learned that people don't like when you go on about eugenics? Or has he had the grim epiphany. all too late, that eugenics is never a good idea? The ambiguity of what happened is never addressed, which means each viewer gets to interpret Kodos as they please: evil of the worst kind, a good man in the wrong place at the wrong time, an unsung hero. (Gotta say, though, dude looks shady as all get out.)
The twist (maybe you were clued in right away that it was Lenore who was killing everyone, but I'm slow on the uptake) is a real gut-puncher. Kodos is horrified and heartbroken that what he thought was his one good and pure thing in the world—his daughter—was tainted by his past; meanwhile, Lenore has become so obsessed with protecting her father that she can't see or understand why he's so upset that she's been killing people to protect his identity. Of course TOS won't let Kodos the Executioner get away with surviving the episode, but his accidental death at the hands of his daughter isn't exactly cathartic or feel-good. Bonus points for Lenore losing her mind over her dead father while in an Ophelia costume. In case you didn't get it.
Admittedly,this is perhaps one of the least science fiction-y stories in all of the TOS episodes; you could recast the story in a totally different setting without changing anything but the smallest details. Everyone watches Trek for different reasons, and if aliens and blasters and new worlds are your reasons (no shame in that!) you're not going to find any of that here.
A minor continuity weirdness quibble: who the heck took Kirk from Iowa to Tarsus IV so he could have seen Kodos with his own eyes, but then took him back to Earth four years later?