The Mirror Universe is one of Star Trek's most popular conceits. Born out of a one-off episode of the Original Series ("Mirror, Mirror"), the "What if humanity hadn't turned out so well?" concept proved to be an enduring one. While Star Trek: The Next Generation never visited the alternate dimension, Deep Space Nine and Enterprise both featured Mirror Universe arcs. Voyager never had one, either, but it did air an episode called "Living Witness" that demonized the crew in much the same way.
Of course, the reason the franchise returns gain and again to the idea of an Evil Twin-verse is that, inherently, it's really fun to take the traditionally goody-goody Federation and give it a bit of a dystopic edge. The Mirror Universe was always a fun vacation from the Federation, because who doesn't want to see staid heroes misbehave and wear sexier uniforms?
Ultimately, Mirror Universe arcs and episodes never amounted to much more than fan service because the concept of the Terran Empire was so implausible it was hard to be frightened of it. It wasn't so much a social commentary as a chance to take a field trip from normal characterization. Star Trek: Discovery has headed in a considerably different direction and has wound up making Star Trek more socially relevant than it's been in decades.
While Discovery's midseason premiere "Despite Yourself" was admittedly hilarious, it was also deeply contemplative, and the following three episodes followed suit. There was little humor to be found in "The Wolf Inside," "Vaulting Ambition" or this week's "What's Past is Prologue." In their place came a sense of despair and foreboding that wafted around Michael Burnham as she ventured deeper into the belly of the beast, and lost every ally along the way.
Instead of being a weightless foray into the realms of "What if?" Discovery took the Mirror Universe seriously. It gave us an unflinching look at what it would really be like to live in a fascist regime built on a foundation of "might is right." "Despite Yourself" ended on a close-up of Lorca's anguished face as he writhed in an agonizer. "The Wolf Inside" featured a visceral execution scene in which criminals guilty of thoughts against the Terran emperor are beamed directly into the vacuum of space. And let's not forget "Vaulting Ambition's"Kelpien soup.
The entire universe is darker, considering the Terran sensitivity to light, the golden tone and long shadows casting everything in an ominous glow. Oh, and virtually no one smiles unless they're about to kill something. No one's twirling any mustaches over here -- they're too busy spending every waking minute trying to make sure they advance at the expense of everyone else. It's hard to have fun when you're constantly watching your back, even you have a crew of slaves at your disposal to give you sponge baths and file your nails.