In the twisted, brutal Mirror Universe, Benjamin Sisko must assume the role of his dead counterpart in order to save the mirror version of his late wife.

Sisko is meeting with Odo and Quark in his office, and is listening to Odo's report about Quark's latest misdeeds, this time preparing with Morn to stage a vole fight. As he leaves from the security meeting, he is abducted by a double of Miles O'Brien, who takes him to the Mirror Universe that Kira Nerys and Julian Bashir visited the year before.

This O'Brien explains that Sisko's counterpart, the leader of the Terran rebellion against the Klingon/Cardassian Alliance, has been killed. O'Brien wants Sisko to pose as his dead counterpart and convince a Terran scientist with the Alliance to join the rebels. That scientist is Jennifer Sisko, the Mirror counterpart of Sisko's late wife. Since she is building a sensor device that will betray the secret location of the rebels in the Badlands, Sisko must convince her to join them before she completes her work or the rebels will have to kill her. Unwilling to watch Jennifer die again, Sisko agrees to intervene. He meets the rebel counterparts of Bashir and Rom, and also Jadzia Dax, who is Sisko's mistress in this universe. The rebel group in the Mirror Universe includes the counterpart of the Vulcan Tuvok, who in the main universe is currently stranded with the rest of the crew of Voyager in the Delta Quadrant.

...
Wikipedia