Discovery pursues Moll and L'ak, sending a shuttle inside a pocket subspace dimension. There, Burnham and Booker find a centuries-old, abandoned I.S.S. Enterprise from the mirror universe, learning something of its history. We also learn something of Moll and L'ak's history, through flashbacks that unfold as they clash with Burnham and Booker. Meanwhile, Rayner takes command of Discovery and must rally the crew to save the away team.
Unfortunately, I didn't feel that "Mirrors" was a very satisfying episode. It teased interesting story threads it chose not to actually pursue, while the story lines it did follow weren't especially compelling.
This is the last season, of course. (Though I hear the writers didn't know that when they were crafting it?) There is a certain logic toward circling back to the mirror universe themes of the first season to bring the story full circle. But this encounter with the I.S.S. Enterprise didn't feel like it amounted to anything. The entire appeal of the mirror universe is seeing the alternate versions of the characters -- watching the actors play against type in a story where anything could happen to them. An empty ship gives us none of that, only serving as a weak bridge between the original "Mirror, Mirror" and the Mirror universe as we saw it in Deep Space Nine.
Indeed, the scenario posed more questions than it answered. If this pocket dimension only exists because of the Burn (they did say that, right?), then how did the scientists get inside to plant a clue there centuries before the Burn happened? If hundreds of refugees from the Mirror universe -- including Saru -- crossed over and settled in the Prime universe, what happened to them? How many people were there two of wandering around? And how can you take us aboard the Mirror Enterprise and not give us a Mirror Spock, Kirk, Pike, or Uhura played by the actors of Strange New Worlds?!
Instead, the episode served to give us a back story I feel like we didn't really need or "ask for," that of Moll and L'ak. It is somewhat interesting to learn that L'ak is a Breen -- to see those aliens rendered on modern Star Trek and see underneath a Breen helmet. (Even if the whole "two faces" aspect was itself somewhat confusing.) But Moll and L'ak have been a mostly off-screen adversary so far -- not even actually appearing in the previous episode, and one of them only appearing in the final seconds of the episode before that. I haven't really become invested in them as bad guys who matter, and so I'm not yet ready to be invested in empathizing with "their side" of the story.
One character who should feel empathy -- it's kind of his thing -- is Booker. But the connection that both he and the writers are trying to force with Moll really isn't working for me. Discovery is usually so "of the moment" when it comes to matters of emotion and representation, that it kind of stuns me that they seem to be unaware of the #MeToo vibes of the dialogue they gave Moll and Booker in this episode. Booker is insisting that his mentor, whose name he took, was a great guy who never did anything bad to him. Moll is saying her experience is that the guy was a thorough villain who treated her horribly -- an experience that Booker just steamrolls over, telling her how she really just has to see the good in the guy. I feel like either this story must be leading toward Booker learning his mentor was not the man he thought, or the writers have shockingly missed the "not very sub" subtext of the story they set up here.
Amid an episode of missed opportunities and uncompelling flashbacks, you just have to subsist on the few moments that did work. Burnham and Booker do make a good team, and the bits of dialogue about them remembering/realizing that played well. The action sequences in Sickbay were pretty fun. And there was nicely subtle writing for Rayner in command -- he could have gone full ham and insisted on doing everything his way, but he really has internalized the lesson of the "time bug" experience to some extent, and tries to meet the crew halfway.
But overall, I feel like "Mirrors" was the most disappointing episode of the season so far. I give it a C+. That said, we've still got two more pieces of puzzle to find, and each of them can come with their own unique adventure that puts things right back on track.