The Lights of Zetar
A collective of alien life forms possess Mira Romaine, a young lieutenant who's won Scotty's affections. But "the lights" won't leave without extreme measures that test Mira's mettle.
This is another awkward move into horror movie territory, with demon possession. Kirk and Spock make limited efforts to communicate with and understand the lights, in contrast to classics like The Devil in the Dark. In fairness, they've always been quick to destroy life forms who are hostile and uncompromising. But the Zetarians are neither potential friends nor galactic threat; it's hard to care about them either way. Mira is a strong character despite her status as victim, and we can see the appeal to Scotty. The smitten engineer has some cute moments, but is sometimes melodramatic. Memory Alpha is a concept not given due consideration, since it hardly matters that the slaughter was at a library. The ending isn't really climactic; we get the sense that this is pretty routine stuff for most officers. And for viewers.
Spock Saves the Day: The first officer does little more here than operate the computers on Kirk's orders and throw out observations on the lights.
Oops: Repeatedly referring to the adult Mira as "the girl" isn't politically correct, but he's not alone in doing so... and it was the '60s.
Developments: Spock can recognize identical brainwave patterns just by looking. Neat. Odd that his mental abilities aren't used to help Mira.