From: Star Trek: Deep Space Nine
Caption: The mirror universe Sisko
Season: 2 Ep. 23
Air Date: 05/15/1994
After experiencing operational difficulties while traveling through the wormhole, Kira and Bashir find themselves in an alternate universe where the space station is populated by exact doubles of Garak and Odo, and is run by Kira's counterpart, Kira II. 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 Odo II. Later, Kira II tells Kira about the last crossover, which occurred with Captain Kirk a century ago. That incident led to the formation of a powerful alliance between the Klingon and Cardassian empires in which Bajor is also a major player. Kira II tells Kira that she cannot allow Kira and Bashir to live, but Kira convinces her counterpart to spare them and let her try to find a way back.
Kira steals a moment with Bashir and tells him what she knows. Since a transporter accident caused the last crossover, he 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 Quark II's help, but he is arrested by Garak, Kira II's aide, for helping Terrans escape the station.
Kira then meets Sisko's counterpart, who receives better treatment than the other Terrans because he runs missions for Kira II. Afterwards, Kira II reassures Kira that she has nothing to fear, and suggests they should become closer. Later, Garak II tells Kira that he intends to dispose of Kira II, and that he will let Kira and Bashir escape if she pretends to be Kira II, then resign and allow Garak II to take over. Garak II then reveals Bashir will be killed if she does not comply.
Kira hurries to Bashir and tells him they must find a way back to the Runabout and make their escape through the wormhole. She fills Sisko II in on Garak II's plan, hoping he will help out of loyalty to Kira II, but he is unmoved. That night, Garak II prepares to put his plan into effect at a lavish party thrown for Kira by Kira II. Meanwhile, Bashir is able to take advantage of an accident at the ore-processing plant where he labors, killing Odo II and escaping.
News of Bashir's escape soon reaches the party, while Bashir manages to locate O'Brien's counterpart, who decides to help this time. However, the two are caught and brought to the party to face Kira II. Despite Kira's pleas, Kira II sentences Bashir to death. When she turns to O'Brien II, he makes an impassioned speech, telling the assembled crowd what Bashir has revealed about a universe where Terrans have respect and dignity. His words move Sisko, who turns on Kira II and helps Kira and Bashir to escape. The two return to their universe, leaving Sisko and O'Brien's counterparts to fight for their rights in their own world.