#9: The Tholian Web
In case you forgot: In search of a missing ship, the Enterprise finds it in some weird in-between space (so much for that sacrifice Lazarus made in "The Alternative Factor") where two different parallel universes are in contact. Kirk gets stuck in the parallel universe and Spock has to decide whether to try and save the captain or to escape this weird bubble in the universe that's making the crew lose their minds before the Tholians finish caging them in.
Much as I love TOS, I have to say: an episode largely bereft of Kirk is almost refreshing to watch. More than that, we've got some good old-fashioned sci-fi going on here, and again, one of my favorite moral dilemmas to come up in TOS: do you risk the many to save the few (or even the one)? Of course TOS always comes down on the side of the few/one, which I guess does get repetitive, but it does make for some of the more compelling plots in the show.
The pacing on "Tholian Web" is solid. Each new disaster (missing the interphase, engines busted, crew going crazy, Kirk's diminishing oxygen supply) adds to the tension without seeing artificially tacked-on, making the payoff when they are able to recover captain Kirk immensely satisfying (even though we know that, of course, Kirk will survive).
McCoy's mistrust of Spock seems to go a bit too far, however. It's been well-determined by now that Spock harbors no ambitions for power whatsoever; Bones refusal to accept this is too stupid to be entirely consistent with his character. I can understand being freaked out by someone's lack of freaking out, but come on Leonard!
Why are the Tholians so needlessly antagonistic? I guess in the big bad galaxy some races are bound to be jerks and it's just too bad that the Tholians are one of them.