Star Trek: The Original Series: Mirror, Mirror

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 the Federation is now called the Terran Empire. The group's uniforms have also changed into more "revealing" outfits, including a decorative sash tied about their waists. Uhura now bares her torso, while Kirk's casual uniform is 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, Mr. Kyle, by a mirror-universe Mr. Spock who now sports a menacing-looking goatee, and a device called an agonizer. The mirror Spock administers the punishment 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 "his own command" if he's freed. 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 5000 colonists on Vega IX.
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 an agony booth and tortures him. Kirk realizes 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.
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 Lt. 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 resisting 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 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 he may have finally found a way back. In a fight with mirror-Spock, 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, but surprisingly agrees to assist Kirk in returning the landing party to their own universe. This gesture convinces Kirk that this Mr. Spock is still an honest and cooperative person 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 Lt. 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."