Obsession
A deadly gaseous cloud drains the red blood cells of an away party, reminding Kirk of a similar incident early in his career. The pursuit soon overtakes him, to the point where others fear for his sanity.
Ack, redshirt slaughter! And Kirk keeps sending them in. His friends have every right to worry about their captain's mental state. The nebulous being is perfect in representing paranoid fixation, while the back story of Kirk's is one of the better ones. (Not another "woman from the past" returning.) Garrovick is hardly captivating but isn't an annoyance either; he simply serves to remind Kirk of his young self. The "ganging up" scene is best, proving not only that Spock and McCoy can work together but, as responsible officers, are quite ready to take over the ship if Kirk has indeed gone mad. But hey, he's right about a lot of the creature's physical properties, so he must be right about everything. The scene with it roaming the ship is exciting, as is the conclusion. Will our noble captain sacrifice himself, as Garrovick thinks? No, that's not the Jim Kirk way. He'll live to die another day. One of the best character-driven stories.
Spock Saves the Day: Spock inquires into Kirk's irrational behavior, takes readings on the creature, and saves Garrovick from it.
Oops: If Spock was trying to give Garrovick a reassuring speech, he was pretty slow in getting to the point. Yet he has no trouble telling Kirk not to feel guilty, in a matter of seconds.
Developments: Usually Spock's green blood just provides another teasing adjective for McCoy, but it comes in handy here. The creature doesn't like his copper-based cells (preferring iron.) The doctor is actually pretty considerate, since I don't believe Spock fully understands obsession and his friend tries to clarify, or at least point to a case study.