This signature episode picks up the premise of "The Alternative Factor" (alternate universes) but actually makes something of it. In addition to being exciting, the resulting episode picks up the exploration of the characters by asking the question, "What would their opposites be like?" It's a simple idea with some very complex overtones. Much later, the same concept would serve DS9 very well and result in some additional classic episodes.
So many things are right about this episode that it's hard to know where to begin. Maybe it's the details which set the tone. From the sword logo plastered all over the ship, to Sulu's horrendous scar, we know that things in this universe are different on the deepest levels. Adding a beard to Spock is a sign of brilliance because it is not exactly an inevitable choice, but so perfectly fits the situation that it becomes the very embodiment of it (and the subject of another rather lame joke at the end). They must have also had fun creating the alternate uniforms.
Shatner soars in a performance that is nuanced and very understated. Kirk quickly realizes the stakes of the situation, but it never seems like a stretch of reasoning. It does not take a genius to figure out what has happened. (A simple premise is an amazing place to start.) Shatner must underplay everything so the truth is not revealed too quickly. He does so masterfully, most notably in the scenes with his yeoman, where intimacy could easily lead to a slip-up.
Similarly, Nimoy makes great work of Alternate Spock. He's given a great script which makes solid claims for an alternate logic which Vulcans could adhere to in a much more brutal system. Nimoy picks this up and adds an iciness to the character which makes his actions entirely plausible. There is no doubt that there are two separate Spocks, but that they share at least some traits -- including loyalty to their commanding officers.
Other great performances are turned in by Nichols and Takei, as well as some leering (and surprisingly passive) bridge guards. Watch their expressions as Uhura distracts Sulu from his console.
The greatness of this episode begins with its script, continues through the performances and details, and culminates in exceptional direction, staging and pacing. Nothing seems forced, and the story is just the right size for the hour. There is action and intrigue, and more wonderful character moments than can be tallied. Special effects are used judiciously, and only the unfortunate denouement feels like some lame humor (and one cheap shot by McCoy at Spock) tacked onto the end.
Rating: Top (2)