The Mirror Universe under the Klingon-Cardassian Alliance

In 2370 a malfunction during a trip through the Bajoran wormhole sent Major Kira Nerys and Doctor Julian Bashir into the mirror universe, a parallel reality not visited by the Federation for about a century. They discovered how much had changed there since Captain Kirk's crossing in 2267.

The mirror Spock had followed Kirk's advice and reformed the Terran Empire, but the gentler Empire proved unable to defend itself and was conquered by an Alliance of Klingons and Cardassians. Terrans had become a subject people, and the Bajorans had risen to power within the Alliance. On the mirror Terok Nor, the ruthless Intendant, Kira's counterpart, governed with the help of the overseer Odo and the scheming Garak, while human slaves worked the ore.

Bashir was sent to the mines, and Kira was drawn into the Intendant's intrigues. With help from the mirror Miles O'Brien, the visitors escaped back to their own universe, but their example inspired O'Brien to lead a Terran uprising. Later crossings by Sisko and others would draw the prime-universe crew deeper into the rebellion against the Alliance.