After fruitless negotiations with the peaceful Halkans on dilithium mining rights on their planet, Kirk, Uhura, Scott and McCoy are beaming back to the USS Enterprise. But due to an ion storm they materialize in a parallel universe on the I.S.S. Enterprise, a nearly identical ship that is governed by brutality. Vice versa, their counterparts from this Mirror Universe find themselves aboard the USS Enterprise of our universe, where Spock quickly puts them under arrest. In the Mirror Universe, the four officers try to fit into their unusual roles on a ship where murder of superiors is an appropriate means to attain a promotion, as Mirror Chekov's attempted assassination of Kirk demonstrates. When Kirk refuses to destroy the planet Halkan, Mirror Spock warns Kirk that he has received orders to eliminate him. While Scott and McCoy are working in engineering on a procedure to return to their universe and Uhura distracts the ship's security officer Sulu, Marlena, obviously Kirk's mistress in this universe, shows Kirk how to get rid of his enemies using a device in his quarters called the Tantalus field. Mirror Spock learns the truth about the landing party through a mind meld with McCoy, and he decides to help them get back to their universe. Marlena, who prefers Kirk over his Mirror counterpart, has to stay behind. Kirk leaves with a plea that Mirror Spock should change something about his universe and points him to the Tantalus field that could allow him to achieve that goal. Back on his Enterprise Kirk meets a newly assigned lieutenant, who is no one else but the counterpart of Marlena in our universe.

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