Kira and Bashir cross into a parallel universe (the same one that Kirk crossed into in the TOS outing "Mirror, Mirror") to find humanity enslaved by the allied Klingons, Cardassians, and Bajorans—ironically enough, because Kirk led the "Terrans" to give up their violent ways, thus leaving them defenseless to the chaos of their neighbors. Kira finds herself face to face with her own counterpart, the evil Intendant Kira, who commands the station.
"Crossover" is a superb episode of unique style and attitude. The extremely neat-looking sets and lighting are dark and brooding. And David Livingston's first-rate direction utilizes freedom with some fresh perspectives; canted camera and low-angle shots add a welcome sense of unreality.
The characterizations can best be described as "brilliant." Nana Visitor's take on the mirror Kira creates a sexy, venomous persona with a healthy dose of narcissism and an intriguing subtext of lesbianism. She's spoiled, ruthless, and fearsome, but still has a respect for life and no love for violence. Meanwhile, Garak is the station's first officer—vindictive and power-hungry. Odo is absolutely hateful and brutal (with "Rules of Obedience" that can earn one a slap in the face for disobeying). Quark turns out to be noble (!), but is executed for helping Terrans escape the slave station. Then there's Avery Brooks' astounding performance as the alternate Sisko—an apathetic, self-serving, charismatic scumbag of a pirate who curries favor with the Intendant by running errands for her (among other things). Mirror Sisko is so watchable for all the wrong reasons; he's truly an antithesis of the real Sisko.
The plot to "Crossover" is fairly simple (Kira and Bashir must escape this universe before they're destroyed by its chaos), with a few interesting plots twists. It's meaty and involving, plunging humanity into the depths of despair. The mirror O'Brien gives the episode a full sense of credibility with a scene that says it all: If history had been just a little bit different, how would he—and all of humanity—have turned out?
That's what makes "Crossover" a real winner. Ask yourself: Based on the way the characters are played in this episode, isn't it possible that the real DS9 characters we know could've been capable of what their counterparts are here—provided history gave them the unlucky draw?