The Mirror Universe version of Miles O'Brien kidnaps Commander 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.
- Becoming the Mask: 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).
- Bed Trick: Mirror Jadzia seduces Ben thinking he's Mirror Sisko, who goes along with it (mostly) to maintain his cover. The Questionable Consent here seems to have only occurred to the writers later, as the next episode where they both appear has her threaten him with a knife if he even thinks about touching her again. (There's no such contretemps with Mirror Kira, but then she's flanderized into an Extreme Omnisexual starting this episode anyway.)
- Bizarre Alien Biology: Quark says that confiscating Morn's voles will break his hearts (yes, plural).
- Continuity Cameo: Tuvok makes a brief appearance as a member of the rebellion, and even gets a few lines.
- Continuity Nod:
- Contrived Coincidence: 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.
- Death Notification: Mirror Kira telling Mirror Jennifer that her husband was killed. She's not terribly broken up.
- Depraved Bisexual: The obvious attraction Intendant Kira showed toward Major Kira in "Crossover" starts getting exaggerated into indiscriminate sexual voracity, a trend that continues through her appearances in later Mirror Universe episodes. Nana Visitor was reportedly not happy, as she'd intended the Intendant's vamping on the Major to be a sign of extreme narcissism, not a normal expression of her sexuality.
- Evil Is Hammy:
Mirror Garak: PURSUUUUUUUUUUUUUUUUUUUUUUUUUUUE!!!
- Nana Visitor and Andrew Robinson are particularly over-the-top in this episode.
- Avery Brooks also breaks out the ham when impersonating Sisko's mirror counterpart.
- Guns Akimbo: Sisko during the escape.
- Impersonating the Evil Twin: 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.
- Indy Ploy: When Sisko and Jennifer get into a firefight in a corridor:Jennifer: Now what?
Sisko: Uh, I'll think of something.
- It's a Long Story: Sisko, when Jennifer asks him who he really is.
- Mandatory Line: The Mirror versions of Odo and Quark were killed in the 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.
- Oh, Crap!: Mirror Kira, when she learns that Sisko changed the access codes to keep her from stopping the Self-Destruct Mechanism.
- Recruitment by Rescue: The whole idea—rescue Jennifer from Terok Nor so she can help La Résistance. (The alternative is to terminate her.)
- Rewatch Bonus: A minor one but it's interesting watching this after the revelation of Prime!Bashir being genetically enhanced as his Mirror Universe counterpart is almost exactly the same despite the fact he'd have grown up a slave without access to genetic engineering. This indicates that maybe all that engineering wasn't even needed and Bashir would have matured into a handsome and smart man all on his own. On the other hand, Alexander Siddig later explained that he played the mirror Bashir as being the antithesis of normal Bashir, describing him as "unstable, difficult, and stupid."
- Self-Destruct Mechanism: Sisko activates it as part of his escape from Terok Nor.
- Series Continuity Error: Alliance ships are shown de-cloaking in this episode. A later episode ("The Emperor's New Cloak") uses a major plot point 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 Expanded Universe attempts to explain this apparent contradiction.
- Talk to the Fist: Sisko punches Mirror Bashir for mouthing off to him, on Mirror O'Brien's advice.
- Title Drop: 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."
- Wham Shot: Mirror Rom's dead body.