"Crossover" is the 43rd episode of the science fiction television series Star Trek: Deep Space Nine the 23rd episode of the second season. It is the first Deep Space Nine Mirror Universe episode.

After experiencing operational difficulties while traveling through the Wormhole, Major Kira and Dr. Bashir find themselves in an alternate universe where the station is populated by exact doubles of Garak and Odo, and is run by Kira's counterpart, who is referred to as 'Intendant'. In this universe, they have no knowledge of the wormhole, and humans have no rights whatsoever. Because of this, Bashir is sentenced to manual labor, working under the sadistic, human-hating alternate Odo. Later, the Intendant tells Kira about the last crossover, which occurred with Captain Kirk a century ago as seen in TOS episode "Mirror, Mirror". That incident led to the formation of a powerful alliance between the Klingon and Cardassian Empires in which Bajor is also a major player. The Intendant says that she cannot allow Kira and Bashir to live, but Kira convinces her counterpart to spare them.

Kira steals a moment with Bashir and tells him what she knows. Since a transporter accident caused the last crossover, Bashir thinks they might be able to escape using another one, and tries to talk O'Brien's counterpart into helping him. Unfortunately, the beaten, put-upon Terran has little interest in risking the wrath of his superiors. Meanwhile, Kira almost succeeds in securing help from this universe's Quark, but he is arrested by Garak – the Intendant's aide, and (unlike the normal universe's Garak) a Gul in the Cardassian military – for helping Terrans escape the station.

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