Alien life seizes control of a Federation colony and its inhabitants, leaving the Enterprise as the last defense from a species of parasitic invaders that induces anguish, madness, and even death in its victims. Among the victims: Kirk's own brother, sister-in-law, and nephew. While investigating on the planet, Spock is attacked by one of the creatures, leaving him in agonizing pain—which he is barely able to control with his Vulcan mental disciplines.
The most appealing aspect of "Operation—Annihilate!" is Spock's struggle of mind over matter—nicely conveyed through Nimoy's balance of wincing, writhing, and snapping back into control. Still, the way TOS plays with the galactic doublespeak can be corny, especially when Kirk and Spock talk about the lifeform coming from another galaxy where "our physical laws wouldn't apply." Come again? Just what galaxy are we talking about? Never mind.
As a body-snatching alien plot, the show is certainly entertaining, if not exactly inspired. I question, however, the wisdom of having an officer say, in regard to one of the fake-looking plastic creatures, "Captain, it doesn't even look real!" Perhaps not, but you never show your cards.