In the sadistic, evil, and brutal mirror universe, Sisko must assume the role of his dead counterpart in order to save the mirror version of his late wife.
Sisko is abducted by a double of O'Brien, who takes him to the mirror universe that Kira and 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 counterpart of Sisko's late wife. Since she is building a sensor device that will betray the secret location of the rebels, which is 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, Rom, Tuvok from the starship U.S.S. Voyager, and also Dax, who is Sisko's mistress in this universe. Only O'Brien knows that Sisko is an impostor.
Terok Nor, the mirror universe's Deep Space Nine, is ruled by Kira for the Alliance. She tells Jennifer that the bloodshed against the Terrans will end once her device is completed. But later, Kira tortures Terran slaves in the ore processing center with the help of Garak, her chief aide. Then, Kira discovers that Sisko is still alive.
With communicators hidden under their skin, Sisko and O'Brien take a rebel ship to Terok Nor. They are immediately captured and taken to Kira, who sends O'Brien to ore processing with the slaves and takes Sisko to her quarters. Later, Sisko meets Jennifer, who despises him, and without revealing his true identity, apologizes for his past treatment of her, then says that he has come to rescue her.
Sisko tries to convince Jennifer that the Alliance is her real enemy, and plans to kill the rebels once they are found. Sensing he may be winning her confidence, Sisko signals O'Brien. In ore processing, O'Brien receives the signal and causes a malfunction that allows him and the Terrans to escape. Meanwhile, Jennifer agrees to leave with Sisko, they meet O'Brien, then head for a waiting ship.
Before they can escape, however, they are cornered by Kira and her troops. While Kira waits for Sisko to surrender, he slips away and herds his entire group into the ore processing center, where they seal themselves within. When Kira, Garak, and her soldiers finally force their way in, Sisko calmly tells her that he has activated the station's self-destruct sequence, which only he can stop. Kira reluctantly agrees to let Sisko, Jennifer, O'Brien, and the Terrans go free in order to halt the imminent destruction. Sisko and the others return to the rebel encampment, where Jennifer admits to realizing that he's not really her husband. She kisses him goodbye, and Sisko returns to his universe just a bit sadder.