TOS Season 2 Episode 4 Mirror, Mirror

It’s the mirror universe episode that started it all, so that counts for something, right? It’s an interesting concept executed in a goofy way, even for TOS standards.

Plot

After failing to persuade the Halkan Council to allow the Federation to mine dilithium crystals on their planet, Captain Kirk, along with Chief Medical Officer Dr. McCoy, Chief Engineer Scott, and Communications Officer Uhura, attempt to beam back to the USS Enterprise during an ion storm, and find themselves on a dramatically different Enterprise designated "ISS Enterprise." First Officer Spock now has a mustache and goatee,[3] sidearms are standard issue, and the ship belongs to an empire rather than the Federation. After inquiring about the status of the mission, Spock orders the bridge crew to prepare a phaser barrage on the Halkans for their refusal to cooperate, then uses an "agonizer" to punish Transporter Chief Kyle for an error.
Kirk orders the landing party to sick bay so that they can assess their predicament in private. He deduces that the ion storm must have opened a barrier between parallel universes, causing the landing parties in each universe to switch places. They decide to impersonate their counterparts until they can find a way home. Uhura ascertains that Starfleet has ordered the Halkans' destruction unless the Empire's demands for dilithium are met. Kirk orders a twelve-hour delay, which Spock reports to Starfleet. Kirk heads for his quarters and is nearly murdered by Chekov and his henchmen, one of whom betrays Chekov and saves Kirk's life.
Kirk's bodyguards arrive and take Chekov to the "Agony Booth" for punishment. When Scott and McCoy join him in his quarters the computer confirms Kirk's hypothesis and provides a procedure to reproduce the transporter accident and return them to their home universe. Kirk also learns that in this universe, his counterpart took command of the ISS Enterprise by assassinating Captain Christopher Pike, and has since committed numerous atrocities. Meanwhile, aboard the USS Enterprise, Spock places their mirror-universe counterparts in confinement, but has not yet determined how to send them home.
On the ISS Enterprise, Scott and McCoy secretly begin work on the engines and transporter. Spock warns Kirk that despite their relationship he cannot support Kirk's aberrant behavior. Kirk defies the warning, releases Chekov from the Agony Booth, and returns to his quarters. There he finds a female officer, Marlena, the "captain's woman"; she assumes his unusual behavior is part of a plot, as their relationship has been cooling of late. Spock interrupts to inform Kirk that he has been ordered to kill him and take command unless Kirk carries out their mission within four hours. Marlena suggests using the Tantalus Field, a weapon concealed in Kirk's quarters. She focuses the device on Mr. Spock, lamenting his imminent demise, but Kirk prevents her from activating it. He reassures her that she is still the captain's woman as he leaves.
In his own quarters, Spock, already suspicious of the landing party, queries the computer about the "classified research" being conducted in Engineering, and decides to confront Kirk again. On the bridge, Uhura distracts Sulu, the security chief as well as the senior helm officer in this universe, from his security board when it signals Scott's connection of warp power to the transporter. Spock intercepts Kirk in the transporter room and leads him at phaser-point to Sickbay, where Scott, McCoy, and Uhura have been waiting. A fight ensues, in which Spock is knocked out by Kirk. McCoy insists on treating Spock, and before they can leave, Sulu arrives with three security guards. Sulu tells them he intends to assassinate Kirk and make it look like Kirk and Spock killed each other, but Marlena intervenes from Kirk's quarters, using the Tantalus Field to vaporize Sulu's henchmen. Kirk renders Sulu unconscious, and Uhura, Kirk, and Scott head for the transporter room, leaving McCoy to follow after tending to Spock. Spock suddenly awakens and forces McCoy into a Vulcan mind meld to learn why the captain spared his life.
Kirk, Scott, and Uhura reach the transporter room to find Marlena waiting. She asks Kirk to take her with them, but Kirk explains that the transporter is set for four people. Marlena points a phaser at Kirk, but Uhura disarms her. They discover power to the transporter has been cut, and Scott can only reset the controls to allow manual operation, requiring one of them to stay behind. Spock arrives and announces he will operate the controls. Kirk uses the time remaining before the landing party must transport out to argue for the overthrow of the Empire, which Spock agrees is inevitable. He urges Spock to take command and find a way to save the Halkans. When Spock reminds him of the necessity of power, Kirk reveals the existence of the Tantalus Field. As he transports them out, Spock promises to consider what Kirk has said.
Kirk, McCoy, Scott, and Uhura beam out, and find themselves back in the Federation universe. Spock reports he found the ruthless attitude of their counterparts refreshing, sarcastically calling them "the very flower of humanity." Kirk is startled by the appearance of Lieutenant Marlena Moreau, recently transferred to the Enterprise, with a report for his signature. In answer to Spock's inquiry, Kirk says only that Moreau "seems like a nice, likable girl" and that perhaps they could become "friends".