This two part episode does take its idea from the T.O.S. episode Mirror, Mirror (Sea2 Ep4). But that's all it does. It is more about the explanation of what happened to a missing Federation ship in Tholian space in the distant future. Now this episode totally takes place in the Mirror Universe. AKA Fringe. No transporter accident here sending the good officers to the bad universe, and vice versa, as in the T.O.S. episode. Just a 2 parter in the bad uni. And a lot of fun with all the cast playing different roles. But what ties this to the future Federation is that In a Mirror Darkly, a future Constitution Class Star Ship the Defiant has somehow just appeared in the present, or its past, and has been captured by the Tholians. For those that remember, you now know what happened to the doomed Defiant that Kirk was trapped on for a while during a phase shift. For those not versed watch the Star Trek episode The Tholian Web (Sea3 Ep9). That explains how the Defiant got here. This future Starship has Warp 9 capability superior weapons and shielding. And The Empire (Federation in the good universe) must have "It" to tip the scales in their war against Good. Remember, here....... Evil must triumph over Good!! Anyway lots of fun watching everyone depart from their original roles. Except T'Pol, who like Spok, is "The same in both Uni's. Logic always wins out. Maybe J.J. Abrams will have one of his sequels of the New Star Trek in the Mirror Uni????