So, it turns out the Mirror Universe is also the Slash fiction universe. Figures. Those mirror universe episodes of Deep Space 9 were somewhat twisted. Still in a little Wikipedia-Shock that DS9 plundered those depths FIVE times.
This episode was of course the very origin of the pop-culture phenomenon of parallel universes inhabited by evil copies of everyone, usually with a beard. Without this there’d be no. . . erm. . . that one South Park, and that episode of Futurama with the boxes.