Like any TOS ep, or any sci fi TV show for that matter, the story takes risks. Either you buy into the premise of the series and the specific plot of this ep, or you simply don't.
There's actually alot to both love and hate in "The Immunity Syndrome". In order to love the ep, you have the get over a few hurdles in order to suspend your disbelief.
- GIANT SPACE AMOEBA - the idea of starships and whole star systems suddenly being drained to death by an 11,000-mile-long single-celled bacteria, free-floating in space, is a bit of a row to hoe. You either accept this or you don't.
- THE DEATH SCREAM THING - The notion that Spock "felt the Interpid die" was a rather weird Vulcan TOS moment. Again, you're either entertained or you're annoyed by it.
- THE ENTERPRISE SOMEHOW PLUNGES INTO THE SPACE AMOEBA. I assume this thing is made of water... and so the Starship Enterprise takes a swim???
I file all of these things under a kind of well-done "flight of fancy" category without any difficulty. I look at "The Immunity Syndrome" as actually being better-done than "Operation: Annihilate!" which had some conceptual similarities. I took "Immunity" as Roddenberry and Coon's salute to
FANTASTIC VOYAGE. (Which, BTW, became its own movie and TV cartoon series.)
Of course, there other minor nit-picks in this one, like here we have yet another Federation starship suddenly and silently biting the dust. (We don't even bother to look for the wreckage this time.) And the horn music used during the first direct probe launch of the Organism is very, very parochial, very 60s. But these turn out to be very minor and can be ignored.
"Immunity" is not the best of TOS. But one great thing about TOS is that there was a cast and crew (then hungry unknowns) who were making a name for themselves by taking risks. "Immunity" as a kind of Roddenberryan FANTASTIC VOYAGE, winds up succeeding despite some stumbles. In the end, though, "Immunity" is like TOS as a whole, you either accept it and enjoy it for what it is or you don't.
I think it was great, but it could've been better.