Bashir and Kira share meditation techniques. Is he hitting on her? He's practically the Captain Jack of Star Trek. No, wait, that's Captain Kirk, silly me. They fly into the wormhole and have some strange turbulance, only to find that DS9 is missing. They locate it orbiting Bajor, and are promptly boarded by Klingons, who are extremely apologetic and escort them back to the station. On the station, they find Garek acting oddly, and evil PVC-fetish-Kira.
Kira and Bashir are all WTF and decide to leave, but evil Kira is clever enough to work it out. She sends Bashir down to to the slave mines, where Evil-Odo is in charge. Evil-Kira reveals to Kira that she's in the Mirror universe. Apparently Evil-Spock was turned good by the experiences of Mirror Mirror, and the Bajoran/Cardassian Alliance was able to crush the Federation due to evil-Spock's reforms.
Kira attempts to talk her evil self into letting them go. When this doesn't work, Kira goes to check on Bashir, who works out the Mirror universe angle as soon as Kira mentions Kirk. Then she visits Quark's bar. It might help her case if she didn't keep pronouncing his name "Quork". She tries to convince him to get her access to a transporter, in return for the ability to send others through to the other universe. Wow, Evil-Garek is really OTT. He has Quark arrested, and Evil-Sisko promptly turns up and serves drinks.
Meanwhile Bashir approaches evil-O'Brien, who's more like cranky-O'Brien, so pretty much the same as in the non-Mirror universe, but doesn't really get anywhere.
Back in the bar, evil-Sisko sexually harasses Kira until O'Brien appears to fix his impulse coil, followed by Evil-Kira calling him to her quarters
Kira finds Evil-Kira having a bubble bath. Evil-Kira says Quark spilled that she was asking about transporters, and they redesigned their transporters so that inter-universe travel wouldn't work. Also, she orders Quark killed.
Evil-Garek approaches Kira with a plan to overthrow Evil-Kira. Kira, however, goes and tells Bashir that she thinks they should try to use the wormhole to get back. Then she goes to Sisko and tries to stir him into action.
Kira and Evil-Kira turn up to a party in the same dress. What a dreadful faux pas!
Meanwhile, Bashir escapes the mines and employs the well-trodden SF cliché of ventilator shafts, where he runs into cranky-O'Brien who guides him to the shuttlebay. Unfortunately they're intercepted and taken to Evil-Kira, who's upset about Evil-Odo being killed during the escape. However Evil-Sisko and his crew help Kira, Bashir and O'Brien escape.
Kira and Bashir take flight in their runabout and nip back through the wormhole to non-evil-DS9.