In an inexplicably fan-favorite romp, the Kirk-enlightens-savages trope is turned on its head, with the savages being alternate-universe Pirate versions of the Enterprise crew.
Schlocky though the concept may be, it is introduced in as compelling a way as could be hoped for. Amidst dilithium crystal negotiations with a society dedicated to total peace, a shaky bit of sci-fi maneuvering finds the good Captain facing his barbaric shadows. The very conflict he faces ideologically is reflected back in the main conceit of the episode.
Unfortunately, any thematic ambitions are restricted to the fringes of this setup. Indeed, it can hardly be praised as an admirable framing device, as the instigating conflict of the peace-lover's negotiations are never returned to or resolved! In the purest instance of concept overshadowing content, the drama quickly devolves into barest needs of the plot.
Arbitrary ticking-clocks abound, a conceptually disproportionate amount of screen time is dedicated to Kirk's fish-out-of-water romance with Marlena Moreau. In light of her brief payoff tag at the end of the episode, the entire relationship plays like an eleventh-hour attempt to wrap the episode up and tie the multi-universe experience together. Likewise, Uhura's seductive distraction of scarred mirror-Sulu turns only on the heels of the plot's needs: returning home.
In an episode almost entirely absent of true, earned conflict, one notable exception rises to the top: Kirk and McCoy elect to endanger themselves for the sake of making the ethical decision by saving the nefarious mirror-Spock's life. This brief moment goes a long way to deliver on the promise of this episode's setup. In the same way he would rather walk away from the negotiations than resort to violence in his own reality, Kirk would risk his escape -- to that same reality -- in order to make the right choice in this one.
In an otherwise well-produced and solidly performed outing, the show has hit a comfortable stride. Though restricted to a limited execution, its ambitions to dramatically explore the darkness innate in every living soul resonated sufficiently with audiences to cement "Mirror, Mirror" as one of the more iconic installments in the show's run.
Just as Kirk reaches for the morally superior position at the expense of his own well being, often the lofty ambitions of Star Trek's thematic scope lie beyond its own ability to portray them.