Star Trek: Mirror Mirror

After failing to persuade the Halkan Council to allow the United Federation of Planets to mine dilithium crystals on their planet, Captain James T. Kirk, along with Dr. McCoy, Scotty, and Lt. Uhura, return to the Enterprise. An unexpected ion storm causes a transporter malfunction, and the landing team beams aboard an unfamiliar Enterprise. 
The group realizes something is amiss as they arrive on the transporter pad: a goateed Mr. Spock viciously tortures the transporter operator, Lt. Kyle, for carelessness in nearly losing the ship's captain. (Low ranking crew members carry "agonizers", which superior officers use to punish them for dereliction of duty.) Kirk deduces that they must have switched places with their mirror-universe counterparts, and the landing party from this universe must now be aboard his Enterprise, and that his landing party must impersonate their counterparts until they can find a way home. In this alternate universe, the USS Enterprise NCC 1701 is called an "Imperial Starship" or ISS Enterprise, and a brutal Terran Empire has replaced the United Federation of Planets. Officers rise in rank by assassinating their superiors (as Kirk discovers when Chekhov nearly succeeds in assassinating him), and as a result all high ranking officers must hire a personal bodyguard. Uniforms in this alternate universe are very different; sidearms and daggers are standard issue, while the clothing itself is much more revealing. Torturing subordinates is an acceptable form of discipline in this alternate universe. 
On board the USS Enterprise Mr. Spock notices the changed personalities of the mirror universe landing party and orders security to take them to a holding cell. The mirror Captain Kirk tries to bribe Spock with rewards of "a command of (his) own" if he's freed but Spock simply replies "fascinating" and continues investigating. Spock comes to the same conclusion as Kirk: the ion storm must have opened a barrier between parallel universes and the two landing parties have switched places. 
Back on the ISS Enterprise, Kirk goes to the captain's quarters on this ship, which are quite different from his own. He discovers that the mirror-Kirk has been ordered to annihilate the Halkans if they refuse the Empire's "request" to mine dilithium, and horrified, Kirk studies his counterpart's records further, In this universe, "Kirk" gained command of the ISS Enterprise by assassinating Captain Christopher Pike and was responsible for massacring 5,000 colonists on Vega IX, among many other atrocities. 
Mirror-Spock informs Kirk that the ISS Enterprise is ready to attack the Halkans. Desperately, Kirk orders a delay in the attack for 12 hours. This piques mirror-Spock's curiosity, but he obeys the order; he does report 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. 
Having failed to sabotage the weapons systems, 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 goes to his quarters and meets the beautiful Lieutenant Marlena Moreau, who refers to herself as the "Captain's Woman". It appears that female crew members may attach themselves by agreement to particular men - Marlena is evidently tiring of her Kirk. Marlena shows Kirk the Tantalus Field, a device in the captain's quarters which can secretly monitor anyone on the ship and "eliminate" them. When he prevents her from eliminating the mirror-Spock, she realizes something is wrong--her Kirk would not have hesitated. 
Kirk stalls the mirror-Spock while his crew search for a way home, but Spock is suspicious. 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. 
Scotty has, with the aid of McCoy, rigged up the necessary connections to make a return switch. Mirror-Sulu, the security chief, is distracted from his monitors at the vital moment of connection by Uhura seeming to encourage his previously-rejected amorous advances. The party reaches the transporter room, but the mirror-Spock confronts them and leads them to sick bay. In the ensuing fight, Kirk knocks the Vulcan and the mirror-Sulu unconscious. (Marlena has "eliminated" Sulu's thugs with the Tantalus Field.) Uhura, Kirk, and Scotty head for the transporter room again while McCoy stays behind to make sure that mirror-Spock is all right. In the transporter room they meet Marlena, who now knows the facts and asks them to take her with them. Kirk refuses on the grounds that the energy is set for four people. Marlena persists and is disarmed by Uhura. 
Mirror-Spock suddenly comes to and quickly mind melds with McCoy in order to read his mind. He discovers the switch, and offers to operate the transporter so that the entire landing party may return to their own universe. This convinces Kirk that this universe's Mr. Spock is still an ethical Vulcan guided by logic. He suggests to mirror-Spock that a Federation-like system is more logical than the ruthless barbarian Empire. Spock objects that one must have the power, and Kirk informs him of the Tantalus Field. Mirror-Spock agrees to consider the idea. (Marlena is presumably now on his side.) 
On board the USS Enterprise, Spock decides to attempt the beaming sequence at the same time the ISS Enterprise attempts theirs. The switch is successful. As the episode ends, 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". The real Spock also comments that the ruthless attitude of the Mirror Captain Kirk, Dr. McCoy, Scotty, and Uhura were refreshing, and "the very flower of humanity".