Mirror, Mirror
Summary
A transporter mishap slips Captain Kirk and his companions into a parallel universe, where the Enterprise serves a barbaric Empire instead of the Federation. Kirk, Bones, Scotty, and Uhura try to act appropriately for their new environment while figuring out how to get back to their universe where their Empire equivalents beamed. In this mirror universe, Mr. Chekov tried to kill Captain Kirk to succeed as Captain but failed and was sent to the torture booth. The Empire Spock receives a command from Starfleet base to execute Captain Kirk if he does not annihilate the planet Halkan by dawn, which he tells Kirk. Bones and Scotty reprogram the transporter to beam them back but are encountered by Spock in Sick Bay. Spock attacks them but is defeated and about to die. Kirk allows Bones to treat him while the others go to the transporter room.
Summary from Wikipedia, CC BY-SA 4.0.
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