“Shattered Mirror” is episode twenty of season four of Star Trek: Deep Space Nine.
Jake arrives back at their quarters to find his father talking to what looks like his mother. Of course, it isn’t his mother, but the Jennifer Sisko from the Mirror Universe. When Sisko is called away to a meeting, he leaves the two talking, but when he returns, they are both gone.
The only thing remaining is an item like the one Smiley O’Brien used to transport between the two universes. Sisko, Major Kira and Chief O’Brien all try to transport across, but only Sisko is transported.
Terok Nor has been captured by the rebels and the Intendant is being held prisoner. A joint Cardassian-Klingon fleet, commanded by the Mirror Worf, is heading to the station to take it back. All they have as defence is a copy of the Defiant – Smiley having, somehow, managed to download the specs to the ship last time he was on DS9 in “Through the Looking Glass”.
They need Sisko’s help getting the ship up and running, because they’ve run into the same problems that the Federation did with the ship’s design – which were never truly fixed. Which is why they lured Sisko across.