On 6 October 1967, Star Trek delivered an episode that remains the gold standard for parallel universes. Half a century later, "Mirror, Mirror" remains a timeless icon of sci-fi storytelling, and one of the best episodes of Star Trek, full stop. Because as cool as the premise is, it's a reminder of what makes Trek's heroes so noble.
The legacy of "Mirror, Mirror" in 2017 is unquestionable. Beyond Star Trek, Mirror Spock's sinister goatee created a pop culture icon, the now-standard visual language for communicating an evil alternate version of a character. In Trek, of course, the Mirror Universe was revisited multiple times — on TV in both Deep Space Nine and Enterprise, giving us glimpses of what the Mirror Universe looked like before and after the classic episode, while countless novels and comics have given us new insights through the lenses of The Next Generation and even the Kelvin Timeline movies.
But "Mirror, Mirror" could've very nearly been nowhere near as impactful as it came to be. Early drafts for the story only featured Captain Kirk being whisked away to a strange, alternate reality, one that was far less sinister than the Mirror Universe we ended up with — instead of showing us what happened to the Mirror counterparts transferred to the "prime" universe. That's something that would have robbed us of the final product's greatest success, which was not just throwing a larger cast of the regular characters into the Mirror Universe (in the actual episode Uhura, Scotty and McCoy all joined Kirk), but seeing their villainous counterparts flung into "our" world, too.
"Mirror, Mirror" was perfectly timed. It isn't an episode that would have worked in Star Trek's first season, when we were just getting to know the Enterprise's bridge crew. It thrives on being able to strongly cast its alternate counterparts into such a wildly opposing characterisation that seeing Sulu clad in security red, stomping about the place, or Chekov so willingly brazen in his attempt to overthrow Kirk's command (and the horrifying price he pays for failing to do so — the agony booth!) is so utterly jarring.
For all the familiarity of the Mirror Enterprise's environs, give or take a few imperial insignia, it's the character work on display that really sells the premise of just how messed up and cruel this alternate reality is. Our heroes are understandably shocked the minute they appear in the Mirror Universe, because of just how alien the actions on display are to them.
But its most effectively chilling work is with the one character who isn't so wildly different from their "prime" counterpart: Spock. Kirk jokes at one point that the Mirror Spock "is very much like our own Mister Spock," but in all seriousness, he's right. While the rest of the cast has a whale of a time vamping it up as their alternate evil selves, Leonard Nimoy's performance is wonderfully understated. Facial hair notwithstanding, Mirror Spock is very much like the Spock we know and love, cool and calculatingly stoic; he just happens to have had his moral core completely flipped. And where the other Mirror versions of the crew thrived on that vast difference to their counterparts, Mirror Spock's eerie familiarity is what makes him such a disconcerting character to watch.
At the end of the episode, Spock Prime informs the returned Kirk and crew that they discovered the bad guys in their midst much more quickly than the crew of the I.S.S. (Imperial Star Ship) Enterprise managed to — because it's much easier for good people to pretend to be monsters instead of the other way around. But what makes "Mirror, Mirror" such a timeless piece of Trek is the moments where the differences between the two, between cruelty and compassion, between unity in division, are much more difficult to spot.
So many happy returns, "Mirror, Mirror". Please don a goatee and be an arsehole for the rest of the day in celebration, everyone.