The USS Enterprise is in orbit of a planet ruled by the peaceful Halkans. A landing party consisting of Captain Kirk, Doctor McCoy, Lieutenant Uhura and Scotty are on the planet to convince the Halkans to allow the Federation mining rights. The inhabitants of the planet, however, are convinced that Humans Are the Real Monsters, and despite Kirk's assertion that if the answer is "no" then they will simply be on their way without troubling the Halkans further, the aliens remain entirely unconvinced.The landing party beam back to the Enterprise. But it all goes horribly wrong, and our heroes find themselves Trapped in Another World. In this Alternate Universe there exists not a peaceful confederation of planets, but instead an Evil Empire that slashes and burns its way across the galaxy. The first and most obvious change is that Spock has got a goatee beard; but there are other signs: nearly all the walls of the Enterprise are emblazoned with a logo of a sword through the planet Earth, and there are also agony booths around, used for punishing misbehaviour among the crew.Finally finding a moment to take refuge alone, our four man crew attempt to figure out what happened. They reason that an ion storm around the planet disrupted the transporter and sent them to a Mirror Universe by accident, swapping them with their own counterparts from this universe. They make a plan to return home, replicating the unique situation that brought them here. But until then they will need to try and blend in as best they can. (As for the unsettling thought of what their counterparts are doing, it later turns out that Spock back in their own dimension has it under control; their barbaric character was immediately obvious to him and he had them hauled to the brig right away.)In his quarters, Captain Kirk meets Marlena Moreau, who introduces herself as "the Captain's woman". He resists the temptation to Boldly Come, instead subtly pressing her for information. Moreau introduces Kirk to the real source of his evil counterpart's power: the Tantalus field, a device which he stole from an alien planet, and which allows him to evaporate people at the touch of a button. Kirk, naturally, resists the temptation to push it and see what happens.Our crew put their plan into action. While Lieutenant Uhura distracts Evil Sulu from his security monitoring board, Scotty shorts out the main phaser couplings and siphons power off to the transporter. Kirk, in the meantime, has to deal with an assassination attempt by Evil Chekov, and another possible attempt from Evil Spock. In the sickbay, a fight breaks out between the crew and Evil Sulu's gang of thugs. Marlena Moreau evens the odds by using the Tantalus field to remove Evil Sulu's gang, and our crew triumph. But Evil Spock arrives on the scene. His curiosity about the crew's bizarre behaviour since returning from the surface has been piqued, and he performs a Mind Rape on the doctor to get the full story.Moreau guides Kirk to the transporter room, but then performs a Heel–Face Turn, deciding that she would rather come back to the prime universe alongside Kirk. To this end she pulls a phaser on him and starts making demands. Captain Kirk Wouldn't Hit a Girl, but thankfully he's not required to, because Lieutenant Uhura sneaks up behind Moreau and instigates a Designated Girl Fight (which she wins). One further obstacle comes in the form of Evil Spock, who arrives on the scene alongside the crew...... but he too has performed a Heel–Face Turn, and receives An Aesop from Captain Kirk:
- Kirk: You're a man of integrity in both universes, Mister Spock.
Mirror!Spock: You must return to your universe. I must have my captain back. I shall operate the transporter. You have two minutes and ten seconds.
Kirk: In that time I have something to say. How long before the Halkan prediction of galactic revolt is realised?
Mirror!Spock: Approximately two hundred and forty years.
Kirk: The inevitable outcome?
Mirror!Spock: The Empire shall be overthrown, of course.
Kirk: The illogic of waste, Mister Spock. The waste of lives, potential, resources, time. I submit to you that your Empire is illogical because it cannot endure. I submit that you are illogical to be a willing part of it.
Mirror!Spock: You have one minute and twenty three seconds.
Kirk: If change is inevitable, predictable, beneficial, doesn't logic demand that you be a part of it?
Mirror!Spock: One man cannot summon the future.
Kirk: But one man can change the present. Be the captain of this Enterprise, Mister Spock. Find a logical reason for sparing the Halkans and make it stick. Push till it gives. You can defend yourself better than any man in the fleet.
Scotty: Captain, get in the chamber!
Kirk: What about it, Spock?
Mirror!Spock: A man must also have the power.
Kirk: In my cabin is a device that will make you invincible.
Kirk: What will it be? Past or future? Tyranny or freedom? It's up to you.
Mirror!Spock: It is time.
Kirk: In every revolution, there's one man with a vision.
Mirror!Spock: Captain Kirk, I shall consider it.
- Agony Beam: Both the Agony Booth and the Agonizer are classic examples.
- Almighty Janitor: Mirror Spock rather enjoys his duties as a science officer, and doesn't terribly want to take Kirk's job because it would mean spending less time on his research and more time having to deal with the burdens of command, to say nothing of how it would make him a bigger target.
- Bare Your Midriff: The female uniforms. Uhura's very toned abs.
- Beard of Evil: The Trope Maker and the Trope Namer. Ironically, Mirror Spock is just about the least evil person in the Mirror Universe, though his logical mindset still masks a sociopathic indifference to human emotion.
- Chastity Dagger: Not that women in the Mirror Starfleet are chaste, but they're quite likely to have to defend themselves against unwanted male attention.
- Continuity Nod: Captain Pike is mentioned, as Mirror Kirk killed him to get his job.
- A Day in the Limelight: For Uhura, along with (Mirror) Sulu and Chekov. Not for nothing is this episode considered one of the show's finest ensemble pieces.
- Deadpan Snarker: Kirk gets a moment when he returns to his quarters after Mirror Chekov tries to assassinate him.McCoy: (touching Kirk's split lip) What's this?Kirk: It's called "blood."
- Description Cut: Kirk ominously ponders what horrors their evil counterparts are wreaking on their Enterprise where they would have full run of the ship. Cut to the prime Enterprise where they are getting tossed into the brig by a thoroughly unperturbed Spock, who apparently identified the doubles immediately.
- Designated Girl Fight: Uhura and Marlena. Doubles as a Curb-Stomp Battle, as Uhura totally owns Marlena from the word "go".
- Disintegrator Ray: The Tantalus Field, the reason Kirk's still in office despite everyone and their dog trying to assassinate him for a promotion.
- The Empire: The Terran Empire, in obvious contrast to The Federation.
- Evil Cannot Comprehend Good:
Spock: It was far easier for you as civilized man to act as barbarians, than for them as barbarians to act like civilized men.
- While the crew from our universe are able to blend in fairly successfully, the Mirror crew who were brought over to the Prime universe were immediately caught.
- Mirror Spock's complete lack of understanding why Kirk left him alive is the reason behind him performing a forced mind meld on McCoy.
- Evil Twin: All of the Mirror Universe versions of the Enterprise crew. The Halkans appear to be about the same, though their Mirror counterparts are more resigned to their fate.
- Exact Words: Captain Kirk tricks Marlena by saying things that are perfectly accurate descriptions of his plans, but are vague enough that Marlena interprets them to mean that he's got an Evil Plan going on.
- Fanservice: Of both male and female varieties. Most notable are Uhura and Marlena in their barely there uniforms, but '60s Shatner had some nice arms.
- First-Name Basis: This is the only Original Series episode in which Scotty calls Kirk 'Jim'.
- Good Scars, Evil Scars: Mirror Sulu definitely has the 'evil' kind.
- Human Aliens: The only thing that differentiates the Halkans from humans is a blue spot on their foreheads.
- Humans Are the Real Monsters: The Halkans believe so. In the Prime Universe, they're wrong. In the Mirror Universe, they're right. Spock has his own take on that subject, of course.Spock: They were brutal, savage, unprincipled, uncivilized, treacherous...in every way, splendid examples of Homo Sapiens; the very flower of humanity. I found them quite refreshing.Kirk (Double Take): I'm not sure, but I think we've just been insulted.McCoy: I'm sure.
- I'm a Doctor, Not a Placeholder: As Scotty needs help, McCoy says "I'm a doctor, not an engineer", but Scotty retorts "Now, you're an engineer."
- Inconvenient Hippocratic Oath: Despite Kirk having had to knock Mirror Spock out when he risked interfering in their plan to return home, McCoy insists on saving him - which would pay off for them in the end.
- In Spite of a Nail: In the alternate universe, there's a repressive Empire in place of the Federation of Planets, but there's still a starship Enterprise with many of the same crew mostly in the same positions. Partly explained by the mechanism of transfer requiring a certain amount of similarity: it could only have happened between two universes which both had a Kirk, a Scotty, a McCoy, and a Uhura doing the same thing at the same time.
- Instant Sedation: Used by McCoy to deal with the guard on the engineering section.
- Kirk Summation: Surprisingly, it actually works.
- Klingon Promotion: Apparently the only way promotion happens in the Imperial Starfleet. Mirror Kirk himself was promoted by assassinating Mirror Captain Pike. Mirror Sulu and Mirror Chekov both attempt this maneuver in the episode itself, impressive considering it takes place over a few hours!
- Mirror Kirk apparently protects himself from hourly attempts to invoke this trope by using the Tantalus Field to kill anyone who might be plotting against them, wherever they are on the ship.
- Knife Nut: All of the Imperial Starfleet officers carry them as standard weapons. Mirror Sulu seems especially fond of his.
- Large Ham: Most of the Mirror Universe inhabitants, but in his brief appearance, Mirror Kirk takes the cake (surprise, surprise...).Mirror!Kirk: I...ORDER YOU...LET ME GO!
- Mind Rape: Mirror Spock takes the capabilities of the mind meld to their worst possible conclusion.
- Mirror Universe: Again, the Trope Namer, as well as the Trope Codifier.
- Unbuilt Trope: This story helped to popularize the Mirror Universe trope, but it didn't take the good-evil reversal as far as later shows would. Spock, despite his mirror-universe Beard of Evil, was "a man of honor in both universes" (though Mirror Spock definitely had a darker personality than his prime counterpart), and the Aliens of the Week, the Halkans, didn't change at all- the only difference with them was that Starfleet wouldn't live up to their fears, while the Terran Empire would.
- More Deadly Than the Male: Marlena doesn't need a knife or a phaser when she can use the Tantalus Field to kill anyone long-distance. Though it should be noted that said field was originally Mirror Kirk's.
- Oh, Crap!: Mirror Sulu's reaction that Mirror Spock has Vulcans in his crew who would avenge him should he "disappear".
- And speaking of "disappearing", when Mirror Marlena uses the Tantalus Field on Sulu's men during his attempt to get rid of both Kirk and Spock, and Mirror Sulu suddenly finds himself outnumbered four-to-one.
- Punishment Box: The Agony Booth again.
- Putting on the Reich: The Mirror Universe officers greet one another with the Roman salute (aka the Nazi salute).
- Red Shirt: One element of consistency between the two universes. Two attempts on Kirk's life lead to five redshirts going bye-bye.
- Rewarded as a Traitor Deserves: When one of Mirror Chekov's henchmen switches sides, he mentions that while Chekov promised him a promotion to chief, Kirk could make him an officer. Our Kirk approves, then belts him to remind him who's the top dog.
- Sensible Heroes, Skimpy Villains: Girls in the Mirror Enterprise bare their midriffs as well as their legs, and Mirror Kirk's uniform does a nice job of showing off William Shatner's nice chest and arms.
- Slouch of Villainy: The normally uptight and well-disciplined Starfleet Redshirts are seen leaning against the turbolift entrance to the bridge.
- Smug Snake: Mirror Sulu. In every one of his scenes he's up to no good, and he doesn't even try to hide it.
- Sure, Let's Go with That: Marlena suspects that Kirk's unusual behavior is part of a scheme to gain rank and power. Kirk plays along with a technically true statement of his intentions that Marlena interprets to mean that he's planning to take over the Empire.
- Tap on the Head: Subverted, hitting Spock over the head with a vase is noted to be fatal without quick medical intervention.
- Villains Blend in Better: Inverted; see Evil Cannot Comprehend Good.
- You Called Me "X"; It Must Be Serious: This is the only time in both the series and the original movies that Scotty calls Captain Kirk "Jim", when he begs him not to stay behind to operate the transporter. It's not lost on Kirk either, who gently but firmly reaffirms his order.