“Crossover” is episode twenty-three of season two of Star Trek: Deep Space Nine.
Doctor Bashir and Major Kira are returning from visiting New Bajor, the first Bajoran colony in the Gamma Quadrant, in a runabout. Just prior to them entering the wormhole, the warp drive misbehaves, and when they exit in the Alpha Quadrant, the station is not there.
Instead, the station is back by Bajor, and two Klingons board their craft after firing at it. When they see Kira, they suddenly become all apologetic for some reason. Back on DS9, everything has changed – but there are some familiar faces. Including a duplicate of Kira, who is called the Intendant, and is in charge of the station and the sector, and Garak is her second in command – and in the Cardassian military.
Other familiar faces are Odo, Quark, Chief O’Brien and Commander Sisko – but they all act differently. Terrans are also treated as slaves, and there are Cardassians and Klingons everywhere. The runabout has entered a different timeline, the one that was originally visited in the original series episode, “Mirror, Mirror”. It seems that original universe Kirk convinced mirror universe Spock to alter things in the Terran Empire in that episode, in order to make them better; unfortunately, this also weakened the Empire to the point where it was conquered by the Alliance of Cardassians and Klingons. As a result of this,Terrans became second class citizens. Admittedly, it wasn’t a nice empire, but the Alliance isn’t much better.
Bashir and Kira have to escape the station and return to their own timeline. Which isn’t easy; Bashir is sent to ore processing, and Intendant Kira is infatuated with her twin. The Alliance also has an existing policy related to the first incursion, and neither will be allowed to escape.
This is the first of a number of episodes of Deep Space Nine in the mirror universe, which is also visited in two episodes of the last season of Star Trek: Enterprise.