Star Trek today tends to have season-long arcs where the only way to fully understand the story is to watch each episode in order; something true of most science fiction and fantasy shows. In the 1960s, with video recorders not exactly a thing in households, people might end up missing an episode with no opportunity to see it again.
So, each episode is basically one story, wrapped up in around 50 minutes and rarely mentioned again.
Anyway, I'm digressing, onto discussing the episode.
On their way to Starbase 6 for a break, the crew of the Enterprise encounter some strange distortion. A Starfleet ship full of Vulcans goes dead, then an entire star system. NCC-1701 must investigate a zone of space darkness that is draining the very energy from the ship - and its crew.
The primary 'triangle' in TOS is Kirk-Spock-McCoy and their overall relationship is very well established by this point. Spock and McCoy insult each other's species as Spock basically says that humans see one death as a tragedy, but a million as a statistic.
(That quote is attributed to Stalin, but there is evidence of similar satirical sentiments being expressed back as far as 1759)
Kirk is weary in the literal and metaphorical sense, the whole grave situation weighing heavily on him. Sure, Shatner does his... famous pauses... but the actor plays Kirk as more than just a cliché. Nimony and Kelley also do very well.
Scotty, Uhura and Chekov also aren't bad. Their material is a fair bit weaker as they're merely serving as plot exposition. Sulu is again absent, his position taken by recurring character Kyle, who doesn't have much too.
The overall mystery is a very strong one, with each layer unwrapped pretty convincingly. Well as convincingly as you can get in a show like this. The solution is a pretty good one as well.
The Netflix version contains the remastered effects, which I'd imagine look much better than the original ones were (although they did win an Emmy); those would probably have taken me out of the episode slightly.
One thing this episode contains in abundance is 'Starship Acting' as the cast throw themselves around the bridge, not always in the same direction. You'd think that they'd invest in some actual harnesses in Starfleet.
A highly enjoyable, self-contained episode with a strong overall plot. Classic Trek.