Operation: Annihilate!
The population of Denova is infected by parasites that take control of the central nervous system. The creatures must die, but appear immune to any known form of destruction.
This is an underrated installment if ever there was one. The unnamed "creatures" here are among the most believable galactic threats in TOS. Some compare them to eggs or parasites, but they most resemble jellyfish, which are at least biological forms. Their method of taking over the human nervous system is explained in gruesome detail, and between Spock and Kirk's son-in-law, we see the pain they inflict. Joan Swift's performance as Aurelan is also painful, but in a different way. Nimoy is quite good as he assures all he's just fine while constantly twitching. The on-location shooting (UCLA? Who knows?) adds some authenticity, though more crowd scenes would also help. Kirk's solution for killing the creatures is brilliant (couldn't resist a pun) but it's a wonder the science officer and doctor don't catch it first. Guess the captain must do something. The whole blindness/eyelid thing is silly, but treated as if everyone knows it is. We can't leave our main characters disabled, can we?
Spock Saves the Day: Spock captures a creature to examine, figures out how they are organized and how they attack, volunteers for tests to kill them, and helps Kirk carry out this method on a planet-wide scale.
Oops: What's worse, going blind or forgetting about one's eyelid?
Developments: The first officer's back to disobeying orders for the good of the ship, though he wins Kirk to side. Some more Vulcan abilities are demonstrated: Spock can keep incredible amounts of pain in check, and withstand "blinding" light. It takes five officers on the bridge to keep him down, and the restraints in sickbay don't stand a chance. (Even with phaser in hand, Scotty's got guts to stand up to him.) McCoy's quite correct in assuming he'd "come up with something" like that eyelid.