Sisko enters the mirror universe and meets a double of his late wife.
The mirror O'Brien kidnaps Sisko into the mirror universe, where the human rebellion has lost its leader, Sisko's double. He is asked to play his counterpart and persuade the mirror Jennifer Sisko, a scientist working for the Alliance, to join the rebels before her work can be used to find their base.
Sisko meets rebel versions of Bashir, Dax, Rom and others, then goes to Terok Nor, run by the Intendant. Rom is killed after being found to be a double agent. Sisko wins Jennifer's trust, O'Brien leads a slave revolt, and Sisko gets her away to the rebels before returning home.