It's superb. Damn, that was good.
It's a parallel universe story, but there are millions of those. Star Trek itself would return to the Mirror Universe many times (DS9, Enterprise, three of Shatner's Star Trek novels, other spin-offs, etc.) What's more, I have a history of hating parallel universes in ongoing SF series, yet I loved this. What's special about it?
Firstly, it's shocking. Our heroes' evil duplicates aren't merely a bit naughty, but genocidal tyrants. Torture and assassination are normal and expected, even against your own shipmates. The medical staff perform experiments on their patients. The Terran Empire enslaves planets and has standing orders to obliterate any world that resists. When Kirk et al are racing against the clock to get back home towards the end of the episode, you're genuinely scared that they might not make it and so might be stuck here.
That's got nothing to do with Spock's orders to kill Kirk, incidentally. Instead it's just sheer gut revulsion at every aspect of this society.
They take this so far! There's a surprisingly detailed look at the position of women in this society, both with Sulu/Uhura and Kirk/Marlena. (Hint: not good.) Sulu's in charge of security, i.e. he's head of the S.S., and he'll make you wish George Takei had played more villains. We see repeated torture, even if it's a bloodless TV-friendly SF variety rather than the Spanish Inquisition. "Your agoniser, please."
The big one, though, is what they're threatening to do to the Halkan homeworld, which we know is peaceful and defenceless. They intend to kill them all... and that's before the opening credits. That shocked me.
Then there's the acting, i.e. Shatner. I love all the main three regulars, but basically it's Shatner. Tell me you've found an actor who brings more conviction to his performances and I'll call you a damn liar. I could spend all day just watching Shatner looking at a viewscreen. He's the one who gives classic Trek its energy and he's on magnificent form here, both as Good Kirk blagging his way through the bastards and as Evil Kirk incapable of understanding decency in the regular universe. I'm not a Star Trek fan, but I'm a massive Shatner fan and he's clearly the best thing in the franchise.
That said, mind you, this is also a huge episode for Spock and, less expectedly, Uhura. I loved the Shatner-Nimoy interaction, in both universes. The Kirk-Spock-McCoy banter at the end is funny, of course. Nichelle Nichols must have adored this episode, although on the downside I was surprised to find James Doohan the weak link among the regulars. (That said, though, he's funny in the movies.)
Unlike In A Mirror, Darkly, this is a universe-crossover episode and so it's specifically about the contrast between our heroes' good and evil versions. Half-hearted or uninspired actors would kill it. We have one of those (Doohan), but everyone else is giving it both barrels... and when one of those is Shatner, you've got something special.
As a Mirror Universe story, incidentally, I also like its impact. There are plenty of Mirror Universe Star Trek episodes, but this feels as if it must surely have done the most to shake that universe's status quo.
Amusing stories I read on the internet:
(a) Star Trek was supposedly banned from showing women's navels, but Uhura's navel is visible in the mirror universe. Apparently this was achieved by taking the Standards representative to lunch during filming.
(b) After watching this episode, some child fans started a letter campaign to get a "Captain's Woman" in the non-mirror universe too. They wanted her to be played by Stefanie Powers. Gene Roddenberry's assistant wrote back, suggesting they ask their parents what a "Captain's Woman" was.
This episode roars. Of course it's still Star Trek and hence basically a laugh. It's never going to be The Wire or I, Claudius. Nonetheless this has so much to love, from McCoy putting everyone at risk to save Bearded Spock to the casually chilling "I press it and he dies". It has a strong moral dimension. I love Kirk's last argument to Bearded Spock. Admittedly at one point I started getting impatient when Kirk was wasting time on endless scenes with Lieutenant Marilyn Monroe (ish), but I suppose "Kirk and girls" is a standard thing? It's an episode with both intelligence and balls. It's showing us powerful morality, but also evil torturing genocidal bastards. Love it.