DS9 Season 2 Episode 23 Crossover

After experiencing operational difficulties while traveling through the wormhole, Major Kira and Dr. Bashir find themselves in an alternate universe populated by exact doubles of the people in their own universe. In this universe, the wormhole is unknown, and Terrans (i.e., humans) are slaves; Deep Space Nine is a mining station run by the Intendant, Kira's counterpart, with Garak as her second in command and Odo overseeing the Terran slaves. As Bashir is sent to work in ore-processing, the Intendant tells Kira about James Kirk's visit to this universe a century ago; that incident led to the formation of a powerful alliance between the Klingons, Cardassians, and Bajorans, and the eventual conquest of the Terrans.
Bashir tries to talk Miles O'Brien, a fellow slave, into configuring the transporters to help Kira and Bashir return home, but the beaten, put-upon O'Brien is unwilling to risk the wrath of his superiors. Meanwhile, Kira almost succeeds in securing help from the bartender Quark, but he is arrested and executed by Garak for helping Terrans escape the station. Kira meets Benjamin Sisko's Mirror Universe counterpart, who receives better treatment than the other Terrans for carrying out piracy on behalf of the Klingon-Cardassian alliance.
The Intendant assures Kira that she has nothing to fear, and suggests they should become closer. Later, Garak tells Kira that he intends to kill the Intendant, and that he will let Kira and Bashir escape if she impersonates the Intendant and yields power to Garak; Bashir will be killed if she does not comply.
Kira tells Bashir they must find a way back to their runabout and make their escape through the wormhole. She tries to recruit Sisko's aid, but he is unmoved. That night, Garak prepares to put his plan into effect at a lavish party thrown by the Intendant. Meanwhile, an accident at the ore-processing plant gives Bashir the opportunity to escape, killing Odo in the process.
Bashir convinces O'Brien to help, but the two are captured and brought to face the Intendant, who sentences them to death. O'Brien 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 the Intendant and helps Kira and Bashir to escape. The two return to their universe, leaving Sisko and O'Brien's counterparts to fight for the rights of Terrans in their own world.