This review contains spoilers.
At first, I thought this episode was going to be a lightweight offering, nothing more than standard than we often sadly became accustomed to in the third season. But the story sort of sneaks up on you, and ends up as one of the far better episodes of the season.
The library, complete with the many android clones of 'Mr. Atoz' (A to Z – get it?) is an interesting setting. But I did feel that the key point of the episode – the arch that transports people back in time (yes, you read correctly!) to not be designed of explained very well, and maybe is the weakest part of the story. And why could Kirk still hear Spock and McCoy if he stood close enough? The whole thread of the transportation is sadly underdeveloped and not fleshed out enough.
But that is a minor point, as the story turns up some great scenarios.
Kirk being accused of being a witch (obviously another very Earth-like planet) is good, if somewhat predictable, but the real pull of this episode is Spock's relationship with the lovely Zarabeth back in the planet's ice age.
Spock is a great character, but I have to confess – dare I say it – I can find him to be very slightly grating at times. But here, Leonard Nimoy is really given something to work with, and this is one of my favourite episodes in terms of Spock's character.
This episode is also unique in that absolutely none of it takes place onboard the Enterprise. We hear Scotty's voice over the communicator a couple of times, but other than the shot of the ship warping away from the supernova in the final moments, the Enterprise is not featured at all here.
A good as the episode is, I did find there to be further plot holes – why (as another reviewer has also commented) did Spock start to devolve but Bones didn't (it can probably be explained away by Spock's more intricate Vulcan past?), and I never fully understood why Spock and McCoy could leave the ice age, but Zarabeth was unable to, unless I missed something. Also, I found it to be a sudden turn for McCoy to reveal Zarabeth as the effective villain, willing to "murder the entire crew" (of the Enterprise to keep Spock there with her; though this can maybe be put down to her doing anything to not have to spend the rest of her life alone in the ice age.
I also found it a bit over-convenient that the trio returned to the location of transport just as Kirk was in the library looking for them.
But ignore those niggles, and this is a highly enjoyable episode. I wasn't expecting much from it, but on hindsight, it is one of the third season's best.