'Mirror, Mirror' the parallel universe so nice, they named it twice!
I kid. It's a terrible place.
Kirk is negotiating for dilithium crystals from the ultra-peaceful Halkan people, who are reluctant to share them for any military application whatsoever.
Beaming back up during a pesky ion storm, Kirk, McCoy, Scott, and Uhura find the ship has changed. Minor infractions are punished with agonizer devices, daggers are standard uniform, and most terrifying of all, belly shirts.
They have to pretend to fit in, but human morality is inverted! Also, the ship is orbiting clockwise. I wouldn't mention it except... how could they possibly... but... SCIENCE!
Scar-faced Security Chief Sulu openly means to force himself on Uhura. Scotty is the only one who can figure a way back, but he's hamstrung by constant surveillance. Because Kirk fails to instantly commit genocide, he is seen as weak and Mirror Chekov tries to assassinate him. McCoy's Sickbay is a cruel chamber of horrors, and the people who work there are self-serving monsters, like the cast of 'Scrubs'. Although here, the Mirror Janitor was kind and sane.
The male-voiced computer confirms the switch can be reversed artificially. It also says Mirror Kirk gained command of I.S.S. Enterprise by killing Mirror Pike, then killed everything else he saw, everywhere he went, ever.
It therefore takes about 9 seconds for our Spock to pop the Mirror versions in the brig back home.
Mirror Spock is still logical, and loyal within reason, but states his beliefs in the effectiveness of rule by terror.
Mirror Kirk's woman Marlena says Spock is the only decent one aboard, and that he will die for it. She reveals the Tantalus device: a monitor system devised by some alien whose lab Mirror Kirk plundered, and using which he has disintegrated over one hundred enemies. It seems as though our Kirk gives her only words of empowerment and a kiss... leaving her alone with a machine that can monitor and kill him instantly. At what stage is too much confidence in one's prowess?
Obviously not yet, since Marlena deletes Sulu's attacking security force instead. Mirror Spock takes the truth from our McCoy's mind. In his bearded wisdom, Mirror Spock elects to let them escape, and Kirk tells him of the Tantalus Field, urging the Vulcan to try to set things right around here. Reform or overthrow the doomed, overreaching Earth Empire.
"One man cannot summon the future." beards Spock.
"But one man can change the present." Kirk replies.
Writer Jerome Bixby brought a very profitable concept into Trek's cannon here. Return visits to the Zone of Lax Values (with accompanying action to stir the blood) have been welcome ever since. It's fun, but don't visit Disneyland over there. Not unless you can stomach Donald's Roast Goofy. Hyuk!