I find it to be a nice change of pace when all the action is set on the ship. Do we have to beam down to a new planet each week? Here we run befoul of a “Zone of Darkness” (the phrase “Black Hole” wouldn’t enter common lexicon until about two years after this episode aired) where everything is all reverse-y. (Oh, Scotty, you seem like such a smart guy — are you really that shocked when the Captain asks you to thrust forward once it is discovered that pushing away only lurches the ship closer?) At the center of the Zone – a thousand mile single cell virus. And it is ready to reproduce (now that it has eaten 400 Vulcans which Spock “sensed” in a scene later stolen directly by George Lucas and Obi Wan Kenobi.) Can the Enterprise act as “antibodies” and save the Universe? Yes — if McCoy and Spock would ever stop bickering. (They both want to be the hero and their usual rivalry borders on a WWF-style smackdown here.) Best of all: the closer to the center of the Zone, the sicker everyone is. And Kirk can only stay awake by shooting himself up with stimulants. By the end of the episode, he’s grinding his teeth and diggin’ on the space age light show. All you need to do is crank the Allmans. Far freakin’ out.
One of my favorite Kirk/Spock/McCoy episodes. The smiles that Bones and Kirk exchange after the good doctor yells at Spock to shut up and be rescued, followed by Spock’s classic sarcastic reply of “Thank you…CAPTAIN McCoy”, are priceless.