People are being killed by an unknown creature at a mining colony, and the Enterprise is called to investigate. It’s a bit more complex than that makes it seem, in a good way. A bit unusual for a Trek episode, in terms of setting/plot, and not the most exciting, but it’s not bad.
Highlights:
focus on Spock
Spock’s emotional concern for Kirk
cute doggyish movements
admirable maternal instinct
Lowlights:
creature costume
Spockism: “It seems logical, Captain. The Horta has a very logical mind. And after close association with humans, I find that curiously refreshing.”