In a Mirror, Darkly is an astounding mix of tropes from The Original Series all viewed through the other side of the looking glass. It's as if the writers/producers had a check list of all the stuff from TOS they hadn't got around to doing and just did it.
The episode is a prequel to Mirror, Mirror but a sequel to The Tholian Web. Thanks to the crew having worn their EV suits over to the Defiant, they all get change into the absolutely fabulous TOS-era uniforms. The old style phasers have a glorious kill setting that straight-up dissolves a guy. There's a dead red shirt on the bridge (exactly where he was in Tholian Web--a nice touch.) Everyone's drinking Romulan Ale and eating Play Dough cubes. Soval has a goatee.
And, oh yeah, a GORN shows up.
Not to mention the totally bitchin' opening sequence:
This is an absolutely beautiful, amazing two-parter. It's chock full of lines and bits that refer to The Original Series and beyond. It is more than worth the watch. It's essential. And, it is the very last romp for My Year Of Star Trek.
Sadly, it was in the shooting of this episode that Enterprise was officially cancelled. At the time that most of this stuff was being filmed, they were still hoping for more. I was still hoping for more. During this viewing, after being reminded of what wonderful, creative things this show was capable of, it just depresses me that Enterprise didn't get a better shot.