"Mirror, Mirror" is an episode of the American science fiction television series, Star Trek (The Original Series). It is a second season episode, #33, production #39, and was broadcast for the first time on October 6, 1967, and repeated on April 12, 1968. It was written by Jerome Bixby and directed by Marc Daniels.

The episode involves a transporter malfunction that swaps Captain Kirk and his companions with their evil counterparts in a parallel universe. In the so-called Mirror Universe, the Enterprise is a ship of the Terran Empire, an organisation as evil as the United Federation of Planets is benevolent.

After failing to persuade the Halkan Council to allow the Federation to mine dilithium crystals on their planet, Captain James T. Kirk, along with Chief Medical Officer Dr. McCoy, Chief Engineer Scott, and Communications Officer Uhura, begin to be transported back to the Enterprise, when an ongoing ion storm causes a transporter malfunction the landing team instead materialize aboard an unfamiliar Enterprise. Their parallels simultaneously materialise on the Enterprise in the Federation universe, and are thrown in the brig by Mr Spock who immediately recognises the changed personalities of the barbarian landing party.

In the mirror universe, the group realises something is amiss as they walk off the transporter pad. First Officer Spock, who now has a mustache and goatee, orders the bridge crew to prepare a phaser barrage on the Halkans in retaliation for their refusal to cooperate, then uses an "agonizer" device to punish the transporter operator for a malfunction. In this alternate universe, the USS Enterprise is called an "Imperial Starship" or ISS Enterprise, and a brutal Terran Empire has replaced the Federation. Officers ascend in rank by assassinating their superiors, uniforms are more revealing, sidearms are standard issue (as well as daggers for officers), and senior officers routinely torture subordinates for indiscipline.

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