This episode is called “The Immunity Syndrome,” but frankly this is a missed opportunity to title an episode “Attack of the Giant Space Amoeba,” or to gleefully over-use the word “entropy.”
Because that’s what this episode wants to be about, folks: entropy, and why it’s the enemy.
This is another one of those times when you can just about see the episode that the episode wanted to be, but it never quite made it out into the world.
What this episode wants to be about:
how fatalism solves nothing except for making extremely orderly log entries
how just because something has worked for a long time doesn’t mean it will always work and how that mindset might indeed get you killed when faced with something totally outside of your experience
how what sets humans apart is our ability to go “wait, WAIT, fuck YOU,” and completely re-invent ourselves in order to survive (the latter being a central thesis statement of Star Trek)
What this episode is actually about:
an up-front death-count on par with a Garth Nix novel
a pseudo-B-plot of inexplicable Spock/Bones dick-swinging over martyring themselves for science
a literal trek into the Space Heart of Darkness whose symptomatic effects on humans include irritability, depression, confusion, and a loss of motivation, all of which translated through the screen to us, your reviewers
As so often happens with TOS, the episode you will inevitably write in your head while watching is far superior to what’s on the screen. And maybe sugar up before sitting down to watch, lest the entropy get you, too.