Mirror, Mirror (Star Trek: The Original Series)

After failing to persuade the Halkan Council to allow the Federation to mine dilithium crystals on their planet, Captain James T. Kirk, along with Dr. McCoy, Scotty, and Lt. Uhura, return to the Enterprise. The effects of an ion storm however, create a mishap during transport, and the landing team beams aboard an unfamiliar Enterprise. 
The team discover they have entered a universe parallel to their own, where the USS Enterprise is called an "Imperial Starship" or ISS Enterprise, and there is no Federation, with a brutal regime called the Terran Empire in its stead. Each high ranking officer is protected by one or more paid personal guards of dubious loyalty, the group's uniforms have changed into more "revealing" outfits, including a decorative sash tied about their waists, plus an assortment of sidearms. A dagger is standard issue for close combat between crew members. Uhura now sports a bare midriff, while Kirk's casual uniform top is replaced by a sleeveless tunic. Kirk also notices he now has a set of strange medals on his chest. 
The group's first experience in this hellish reality is the unforgiving torture of the transporter operator, Lt. Kyle, by a mirror-universe Mr. Spock who now sports a menacing-looking goatee. Each low-ranking crewman wears a device called an agonizer, which is to be surrendered to a superior officer on demand for punishment of said crewman for failing in their duties. The mirror Spock administers the punishment to Lt. Kyle for almost losing the Captain to the operator's carelessness. Almost immediately, Kirk guesses that the mirror-universe landing party must have beamed aboard his Enterprise but the crew here does not know this. The team decides to impersonate their mirror counterparts for now, until they find a way to return to their universe. 
Fortunately, their fears of what their counterparts could be doing in their universe are groundless. Back on the USS Enterprise Mr. Spock immediately discovers the personae of his Captain and the rest of the landing party have changed significantly and orders security to take them to a holding cell. The mirror Kirk tries to bribe Spock with rewards of "a command of (his) own" if he's freed, among other offers. Spock simply replies "fascinating" and goes about his investigation into what has happened. Spock determines that the ion storm must have opened a barrier between parallel universes, and somehow, the landing team's counterparts switched places. 
Back on the ISS Enterprise, Kirk visits his quarters and finds it very different from the one he has become familiar with. There he examines his mission orders learning that he has been ordered to annihilate the Halkans if they refuse the Empire's "request" to mine dilithium. Horrified, Kirk studies his counterpart's records further, learning he succeeded command of the ISS Enterprise by assassinating Captain Christopher Pike and that he also was responsible for massacring 5,000 colonists on Vega IX, among many other atrocities. 
Mirror-Spock informs Kirk that the ship is ready to attack the Halkans, and Scott reports that he failed to sabotage the weapons systems. Desperately, Kirk orders a delay in the attack for 12 hours. This piques mirror-Spock's curiosity, but he obeys the order. Kirk then survives an assassination attempt by the mirror Mr. Chekov, and Spock subjects Chekov to torture within an agony booth as punishment. Kirk surmises that the fastest known path to promotion in the parallel universe is to successfully kill one's superior officer, especially when they appear to be neglecting their duties, especially "prime orders of the Empire" as Chekov notes. 
Meanwhile, Scotty and McCoy work secretly to figure out what happened with the transporter. While Scotty is finding a way to return them to the correct universe, Kirk meets the beautiful Lieutenant Marlena Moreau, who refers to herself as the "Captain's Woman." At the same time, mirror-Spock reports the suspicious activity of his Captain to the Imperial Command, and receives orders to kill Captain Kirk if he does not carry out the order to destroy the uncooperative Halkans. 
Back in Kirk's quarters, Marlena shows Kirk the Tantalus Field, a device which can secretly monitor anyone on the ship and "eliminate" them at his leisure. When he prevents her from eliminating the mirror-Spock, she realizes Kirk's personality is different. 
Kirk continues to stall the mirror-Spock until his team can find a way back to their reality. Spock, not wanting command of the ISS Enterprise as it would make him an instant target of assassination, decides instead to study the Captain as long as he can. Marlena wants her Kirk back as well and helps the team return as much as she can. 
Scotty reports to Kirk that there may be a way to return to their universe, but one of them would have to remain behind and manually operate the transporter. The mirror-Spock suspects the landing party are conspiring for some reason, and confronts them. In the ensuing fight, Kirk knocks the Vulcan unconscious. When mirror-Sulu and his thugs attempt to kill the landing party, Marlena eliminates the thugs with the Tantalus Field and Kirk knocks out mirror-Sulu. Uhura, Kirk, and Scotty head for the transporter room while McCoy stays behind to make sure that mirror-Spock is alright. Mirror-Spock suddenly comes to and quickly mind melds with McCoy. He discovers the switch, and subsequently offers to operate the transporter to return the landing party to their own universe. This gesture convinces Kirk that this universe's Mr. Spock is still an ethical Vulcan guided by logic despite the current environment. He suggests to mirror-Spock that a Federation-like system is better and more logical than the ruthless totalitarianism of the Empire. Mirror-Spock agrees to consider the idea and is encouraged when told about the Tantalus Field he could use. 
Meanwhile on board the USS Enterprise, Spock decides to attempt the beaming sequence at the same time the ISS Enterprise attempts theirs. He has the mirror-universe landing party take their positions on the transporter pads and prepares to beam them out. The transport begins at the same time the Imperials beam their party out. The exchange is successful, and the landing party is repulsed when they learn about their counterparts' barbaric personalities. 
Later, back on the bridge, Kirk meets his own universe's Lieutenant Marlena Moreau, who is quite a different girl from what he experienced her to be in the other universe. Kirk tells Spock that Moreau "seems like a nice, likable girl" and that he thinks they "could be friends."