“Through the Looking Glass” is episode nineteen of season three of Star Trek: Deep Space Nine.
Commander Sisko is in Ops when Chief O’Brien arrives and abducts him at gunpoint, forcing him into the transporter. Only this isn’t Sisko’s Chief O’Brien, but ‘Smiley’ O’Brien from the Mirror Universe from “Crossover”.
This O’Brien wants Sisko to take the role of his Sisko, the latter having been killed when his ship was destroyed during an attempted attack on Terok Nor. The rebellion, which Sisko was leading (and a rather puny rebellion it appears to be currently) was trying to stop a Terran scientist from developing new scan technology that would allow the Cadassian-Klingon Alliance to scan the Badlands where the rebellion’s forces are hiding.
Sisko is not at all interested, but O’Brien explains that he has the best chance of persuading the scientist to change sides, as she’s his, admittedly estranged, wife. If she can’ be persuaded, she will need to be killed. Sisko becomes rather more interested when it turns out that the mirror Sisko’s wife is actually Jennifer, his own dead wife who was killed at Wolf 359.
There are familiar faces from DS9 in new roles, just as in the previous episode, but there’s also a new one – Tuvok, from Voyager, makes an appearance too.