This is one of the better episodes of season 3. After having set up Spock as emotionless, the writers keep trying to circumvent it and have him engage in romance, using everything from weird alien spores to body-controlling telekinesis to just plain inexplicable character derailment. But this is probably the most sensible and satisfying Spock romance story - in fact it's very much Spock's "City on the Edge of Forever" moment, as it also involves time travel and having to leave a woman in the past.

There are a few niggles: The whole mix-up with the time portal is dependent on Atoz not giving Kirk a straight answer, and possibly moreso on Kirk not bothering to ask a straight question. The idea that Spock travelling 5000 years into the past would affect his emotional stability because Vulcans of that time were barbaric is interesting, but it's unexplained, and there appears to be no effect at all on McCoy's mental processes.

But I can forgive those for this shining light in the drek of season 3. Next week, the last episode of The Original Series!

Transcript

1
Captain's log, Stardate 5943.7. Beta Niobe will go nova in exactly 3.5 hours. Rather than depart, we are beaming down to see if we can get into trouble.
Spock: Scans show the population has mysteriously vanished.
McCoy: Maybe it was a PLAGUE!
2
Atoz: We're Mr Atoz. Welcome to the Library. But you must get to safety!
Kirk: Where did everyone else go?
Atoz: To safety! Duh!
3
Atoz: Ask me some relevant questions and I might explain.
Kirk: No, we'll remain confused and browse your collection of vintage CDs.
Voice from through a portal: Help!
4
Kirk: A woman in trouble! Here's my chance!
McCoy: {following} Jim!
Atoz: No, wait!
5
{an old town}
Kirk: Hands off her, you villains!
Fop: She's a cutpurse!
McCoy: {off-panel} Jim! Where are you?!
Cutpurse: And ye're a witch!
6
{an ice age}
Spock: Captain, we are in some sort of ice age.
Kirk: {off-panel} Get back to the Library!
Cutpurse: {off-panel} Witch!
7
{a cave}
Zarabeth: I'm Zarabeth, welcome to my cave, where it's warm enough for me to wear just a leather bikini.
Spock: I am Spock, and you are beautiful.
8
{Spock and Zarabeth kissing}
McCoy: Spock! We have to go back and find Jim!
Spock: Yeah... You go...
9
{the Library}
Atoz: You're back!
Kirk: The Prosecutor sent me back when he learnt I was from the future. Now show me CDs of the ice age!
10
{the cave}
McCoy: Spock! We're 5000 years in the past! Vulcans are savage emotional barbarians!
Spock: {gripping McCoy by the neck} So are humans!
McCoy: Oh, good point.
11
{out in the snow}
Zarabeth: Don't go, Spock!
Kirk: {off-panel} You have to come through together, or the time portal won't work!
Zarabeth: Go.
12
{space, the Enterprise flees a huge explosion}
Caption: Reunited, they warp out seconds before the star goes nova.
Kirk: All's well that ends well!
{Spock in inset, sad}