Mirror, Mirror
A transporter accident sends Kirk, Scotty, McCoy, and Uhura to a mirror universe, where they must carry out orders from the Earth Empire and interact with twisted versions of their shipmates.
This is one of the most influential episodes on pop culture: from its air date onward, all that is needed to denote an evil character is a goatee. Spock's infamous beard is just one of many delightful details seen throughout the mirror ship. Furthermore, there's simply more happening here than on many TOS adventures. There are twists, double-crosses and subplots carefully interwoven. Spock is the only "counterpart" with the capacity for change, but that might differ if Kirk had more time for inspiring speeches. We don't see much of the evil landing party, but Mirror Kirk seems more smarmy than we're accustomed to. Our Kirk seems more confident than usual with Marla, but this might be because it's a deception. The Tantilus Field remains a bit mysterious, but all the other torture devices are straightforward. Most disturbing, however, is the first forced mind meld. McCoy still looks shaken in the transporter room. But Kirk knows Spock in either universe and how to convince him of the need for change. Mission accomplished.
Spock Saves the Day: Default Spock is quick to spot the switch and take efforts to get everyone back to their proper universes. Mirror Spock realizes order must be restored, and that the Empire is doomed.
Oops: Spock has curly hair in the sickbay fight! With Nimoy doing most of the tough moves and slamming into the wall, some bizarre contract with the Stunt Doubles Union can be the only explaination.
Development: It's difficult to assess Mirror Spock because though he is in many ways the same Vulcan, he's also, well, not. We don't know all that's shaped him, and can't assume it's exactly the same guy under different cirmcumstances. He's notably lacking Spock's dedication to peace, compassion, understanding - we're left to wonder how Vulcans fare in this universe. They're still logical, but can't be very virtuous. Notice Kirk convinces Spock to sabotage the Empire out of self-interest, not higher ethics. Perhaps Mirror Spock did pick up some understanding from his meld McCoy. This scene foreshadows Spock's "mind rape" of Valeris in The Undiscovered Country.