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.

* 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).
* BizarreAlienBiology: Quark says that confiscating Morn's voles will break his hearts (yes, plural).
* ContinuityCameo: [[Series/StarTrekVoyager Tuvok]] makes a brief appearance as a member of the rebellion, and even gets a few lines.
* ContinuityNod: Quark tries to explain the voles in his and Morn's possession by saying that vole infestations are common on [=DS9=]. [[Recap/StarTrekDeepSpaceNineS02E17PlayingGod Yeah, been there, seen that.]]
* ContrivedCoincidence: There's no in-universe explanation why Mirror Dax and Mirror Bashir just happen to be among Mirror Sisko's crew, they seem to be there just so Terry Farrell and Alexander Siddig would have something to do in the episode.
* DeathNotification: Mirror Kira telling Mirror Jennifer that her husband was killed. She's not terribly broken up.
* DepravedBisexual: The obvious [[ScrewYourself attraction]] Intendant Kira showed toward Major Kira in [[Recap/StarTrekDeepSpaceNineS02E23Crossover "Crossover"]] starts getting [[{{Flanderization}} exaggerated]] into [[AnythingThatMoves indiscriminate sexual voracity]], a trend that continues through her appearances in later MirrorUniverse episodes.
* EvilIsHammy: Nana Visitor and Andrew Robinson are particularly over-the-top in this episode.
* GunsAkimbo: Sisko during the escape.
* ImpersonatingTheEvilTwin: The Mirror Sisko isn't exactly "evil", but he's certainly more of a bastard than the regular Sisko is. Jennifer notices the difference and eventually figures out that this Sisko is not her husband.
* IndyPloy: When Sisko and Jennifer get into a firefight in a corridor:
--> '''Jennifer:''' Now what?\\
'''Sisko:''' Uh, I'll think of something.
* ItsALongStory: Sisko, when Jennifer asks him who he really is.
* MandatoryLine: The Mirror versions of Odo and Quark were killed in the [[Recap/StarTrekDeepSpaceNineS02E23Crossover previous]] Mirror Universe episode, so Creator/ArminShimerman and Creator/ReneAuberjonois 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.
* OhCrap: Mirror Kira, when she learns that Sisko changed the access codes to keep her from stopping the SelfDestructMechanism.
* RecruitmentByRescue: The whole idea--rescue Jennifer from Terok Nor so she can help LaResistance. (The alternative is to terminate her.)
* SelfDestructMechanism: Sisko activates it as part of his escape from Terok Nor.
* SeriesContinuityError: Alliance ships are shown de-cloaking in this episode. A later episode (The Emperor's New Cloak) establishes that the Mirror Universe doesn't have cloaking technology. This also contradicts the Enterprise episode "In A Mirror Darkly", where the NX-01 has a Suliban cloaking device. There are various ExpandedUniverse attempts to explain this apparent contradiction.
* TalkToTheFist: Sisko punches Mirror Bashir for mouthing off to him, on Mirror O'Brien's advice.
* TitleDrop: Mirror O'Brien provides it when he tells Sisko where they've beamed to.
--> "I guess you could say we just stepped '''through the looking glass'''."
* WhamShot: Mirror Rom's dead body.