This is the third in a series of "Mirror Universe" episodes on DS9. In this one, the Mirror humans kidnap Jake Sisko in order to lure Captain Sisko over to their universe. They need Sisko's expertise in order to build a Mirror Defiant; the ship is needed to defend the humans against the vengeful Klingon-Cardassian Alliance.
One of the threads running through the episode is Jake's affection for his (almost) mom, Mirror Jennifer Sisko. It's an interesting idea but its execution didn't have much emotional resonance for me; others may feel differently.
The best part of the episode, in my opinion, is the space battle that takes place near the end. I think the two previous "Mirror" episodes, "Crossover" (season 2) and "Through the Looking Glass" (season 3), are better than this one.