Jennifer Sisko from the mirror universe visits Captain Sisko's quarters. Jake Sisko, although he knows Jennifer is not his mother, becomes very affectionate to her. Benjamin is then called to duty, and he is forced to leave the two alone.
When he returns, he discovers they have left the station, leaving behind a Multidimensional transporter device. Sisko tries to follow this trail, accompanied by Major Kira and Chief O'Brien, only to discover the device was programmed to transport him to the mirror universe alone.
On Terok Nor, Smiley explains they have built a copy of the Defiant based on the data and schematics that Smiley downloaded from Deep Space 9's computers. Like the original Defiant, this copy has structural problems, and Terrans need Sisko's help to prepare it for an upcoming battle with the Alliance. Smiley is willing to transport the Siskos back to their universe before the battle begins, but only after the modifications are complete. Benjamin agrees to help them and starts working hard. Meanwhile, Jake spends most of his time with the counterpart of his mother.
The Alliance attack, commanded by Regent Worf, is expected in four days--likely not enough time to finish the modifications to the Defiant. To stall for time for the much needed repairs, Bashir and Jadzia Dax pilot a smaller ship to intercept the Klingon ships. Jennifer approaches Sisko and offers to send Jake back home immediately, even though Sisko has not yet finished the repairs he promised. Sisko agrees that Jake should get home to his universe as quickly as possible. As Jennifer and Jake make their way through the airlock, they are trapped by Kira. Kira shoots Jennifer, but spares Jake's life once she realizes that he is Sisko's son. She tells Jake to give his father a message--that Sisko is now in her debt for sparing his son's life, and it is a debt on which she intends to collect. A short time later, Jennifer dies in the infirmary.
Upon suggestion of Garak, Worf opts for a faster transfer and arrives on the second day of repairs, while the Defiant's modifications are barely installed. Sisko then resolves to command the Defiant himself, which Smiley gladly accepts. In battle, the Defiant makes quick work of the smaller Klingon Birds-of-Prey and inflicts heavy damage on the larger Galor-class cruisers. Although the small Defiant was heavily outgunned by the Regent's flagship, under the piloting of Captain Sisko the Rebel ship inflicted major damage on the massive warship while avoiding most return fire. The small ship commanded by Bashir and Dax arrives just in time to assist the Defiant in the skirmish, and together the two ships force the Regent's fleet to abandon his goal of taking Terok Nor and retreat from the battlefield.