During an ion storm over an alien planet, there is transporter malfunction on the Enterprise. The landing party of Kirk, McCoy, Scotty, and Uhura find themselves in a mirror (parallel) universe aboard a ship run by a ruthless crew. Their evil counterparts take their places on the real ship. Our heroes must find a way back before they’re discovered by a crew (where using treachery, violence, and seduction are common)! Kirk also must avoid destroying the peace-loving Halkans, who refuse to deal w/ the Empire (b/c of the power for destruction that their dilithium crystals would give them).
The Mirror Universe salute may remind modern viewers of the Nazis; however, it comes from Ancient Rome. One the first things you’ll notice is the clothing, accessories, and hair (esp. the mustache and goatee on Spock). TOS was usually not allowed to show women’s navels, but Uhura’s toned abs are visible. They filmed while a PA took the Standards representative to lunch- LOL! Sulu wears a red shirt as he has a different role in Security; he has a long scar on one cheek. Phasers are worn upside-down on the left hip; I only noticed this on my third viewing.
Kirk (to fit in w/ his environment) warns the Halkan commissioner: “We will level your planet and take what we want- that is destruction: you will die as a race.” Kirk is faced w/ more complication when he discovers a gorgeous woman waiting in his quarters- Lt. Marlena Moreau (Barbara Luna- an actress w/ Latina and Filipina heritage). She is clever, opportunistic and feisty; she desires to be “the woman of a Caesar” (another call-back to Ancient Rome). Marlena gets suspicious b/c this Kirk is a lot less brutal and insensitive. After filming had begun, Luna was diagnosed w/ strep throat. Since the script called for Capt. Kirk to kiss her, they had to postpone that scene for three weeks until she was well.
This is one the most popular TOS eps; also, The Mirror Universe was later depicted on Star Trek: Deep Space Nine and Star Trek: Enterprise. Ronald D. Moore (a writer/producer on later Trek series) cited this episode as one of his favorites; he is also the creator of the rebooted Battlestar Gallactica. The writers examine the negative side of the main characters, which is fun and unexpected. While Spock remains mostly true to the Spock we know/love, Sulu is a cunning schemer and young Chekov turns out to be a risk-taker. I also discovered that there is a second Vulcan serving on the ship (as Mirror Spock’s security guard).
 Instead of the usual prime directive and the overriding desire to do good, this anti-Enterprise world is just plain awful and a great counter-point to the sometimes saccharine-like world of Star Trek where everyone gets along a bit too often!
 …there’s something about tapping into the dark side of all our beloved characters here which makes this an irresistible mix of tension and adventure – an ultimate Trek, if you will.
 The acting takes it over the top. William Shatner’s Kirk displays the quick wits and cleverness that make the character so interesting. And notice how our good guy Kirk is not entirely uncomfortable in his new, dangerous environment. Leonard Nimoy’s evil version of Spock is genuinely menacing in a cool, calculating way. Nichelle Nichols’ Uhura shows us a cunning, wily side… But the acting prize goes to George Takei. …his evil Sulu is slimy, sleazy, scary, and wonderfully despicable.
-Excerpts from IMDB reviews