Entering the operations center, Chief O'Brien asks for a bit of Commander Sisko's time. Before anyone can stop him, O'Brien places a phaser to Sisko's back and transports him off of the station. There, O'Brien explains that he is the mirror universe O'Brien. Sisko is needed to take the place of the now deceased mirror universe Sisko in order to keep the rebellion against the Alliance alive.
When the Cardassian occupation of Bajor ended in 2369, the mining space station Terok Nor was left abandoned, its systems ripped out. By invitation of the Bajoran provisional government, Starfleet stepped in to oversee the rebuilding and day-to-day operations of the newly christened Deep Space Nine.
The years of Starfleet's nonintervention into Cardassian matters, however, had taken its toll on relations with the Bajorans. Consequently, Starfleet's position was a tenuous one; many Bajorans, suspicious and hostile as a result of decades of Cardassian oppression and brutality, did not greet the Federation's presence with open arms. The Federation's assistance is put to the test as DS9 becomes a center of travel and commerce that stretches into the Gamma Quadrant.