Spock, McCoy, and Kirk become trapped in a planet's past with time running out until the world's star goes nova.
The storyline here works well enough that I forget the problems I have with some details - this is probably my first or second favorite episode of Season 3.
The idea that a planet's people would retreat to their own past is really fascinating, the set of "the library" is fun, and the writing does a good job setting up a sense of past, present, and tragedy in all times. Things that bother me are the idea that the planet's "middle ages" are so much like Earth's and the fact that individuals have to be "prepared" to live in the past. Plenty of previous episodes have time travel without being altered by a machine and it doesn't add much to the story. The concept is important in Spock somehow "reverting" to a savage state, but this makes no biological sense anyway. For once, the writers could have just had the Human/Vulcan Spock fall in love because he meets someone. If the writers just HAD to have a plot device, they could have made up some mumbo-jumbo about how all Vulcans (of any time) are in psychic contact (as told in "The Immunity Syndrome") - or better, just forget it. Finally, and rarely mentioned, is the fact that the time paradox here is immense, this world's people are literally all their own ancestors.
But, I'll overlook all this for a good story with good lines. The Ice Age scenes are great, featuring the drama of time running out and memorable lines from McCoy ("my life is back there, and I WANT that life!"). Zarabeth's baleful glance back as Spock and McCoy return to the library has a ton of pathos. The music, while not original, heightens the sadness. The best of Season 3 was characterized by exploring the human condition, and this one rings the bell.