The Mirror Universe version of Miles O'Brien kidnaps Captain Sisko and transports him to his universe. Turns out the Mirror Sisko has been killed, and the other Sisko has to impersonate him in order to complete a mission for the Rebellion that he left unfinished. The reason Sisko is needed is because in the Mirror Universe Jennifer is alive and working as a scientist for the Alliance. Unless Sisko manages to convince her to change sides, the technology she's working on will be used to crush the Rebellion once and for all.
* ContrivedCoincidence: There's no in-universe explanation why Mirror Dax and Mirror Bashir just happen to be among Mirros Sisko's crew, they seem to be there just so Terry Farrell and Alexander Siddig would have something to do in the episode.
* BecomingTheMask: Despite his initial reservations, Sisko seems to enjoy impersonating his far less moral Mirror Universe counterpart. For example, he appears to have no problems having sex with both Mirror Kira and Mirror Dax, despite the fact that his relationship with both is completely Platonic in his universe, and that he's been their friend and colleague for years (or decades, if you count Dax's previous host).
* ImpersonatingTheEvilTwin: The Mirror Sisko isn't exactly "evil", but he's certainly more if a bastard than the regular Sisko is. Jennifer notices the difference and eventually figures out that this Sisko is not his husband.
* MandatoryLine: The Mirror versions of Odo and Quark were killed in the [[Recap/StarTrekDeepSpaceNineS02E23Crossover previous]] Mirror Universe episode, so Armin Shimerman and René Auberjonois aren't seen once the story moves to the [=MU=]. Quark and Odo only appear briefly before the opening credits, discussing a mundane matter that has nothing to do with the rest of the episode.