The plot is Smiley breaks into the prime universe and conscripts O’Brian to come back with him and fix the beaten-to-shit ISS Defiant for one last battle to save the newly-democratic Terra.
I hate the mirror universe with a passion. It is by far my least favorite plot device in all of Trek.
Plot device? It has less than nothing to do with actual plot elements within any of the shows. Usually it’s just a fun “what if” kind of thing ranging from moral philosophy to just having a fun romp.
I disagree. But regardless I still don’t like it.
Why do you disagree?

While I don’t necessarily disagree with your characterization of the mirror universe as a “what if “. I do disagree with your characterization of it as not a plot device, and with your characterization of “what if”s in general as not plot devices.

For instance, several of the mirror universe episodes revolve around characters that are dead in one universe or the other, or have otherwise been incapacitated. Thus the plot could not exist without the mirror universe existing. In my opinion that pretty squarely makes it a plot device.

Its a story device, definitely, but one that is self-contained. It does not impact the plot of the shows overall. It helps with character depth, which is something I think the 90s shows did exceptionally well. But that’s it’s largest impact, and thus is as meaningful a plot device as any other throwaway storyline from a particular episode. Its only defining trait in that regard is that the DS9 writers made it a recurring one.

What about it as a “plot device” do you think is contrived or lazy? Or is there some other aspect that you find distasteful about it?

and thus is as meaningful a plot device as any other throwaway storyline from a particular episode

Regardless of how much you diminish it, it’s still my least favorite of whatever category you want to put it in.

Mostly, it has to do with that each series of Star Trek, at least from the 60s to the 2000s, were vastly rooted in the best science that day could offer. These are shows of science fiction not science fantasy. One might even go as far as to call them speculative fiction, though that can be debated. But then out of nowhere, we’re throwing in the facial hair evil twin universe in the story line. Even with DS9 trying to dress it up a bit still left me with a bad taste in my mouth. They, to my memory, never tried to explain it like somehow our universe was split Dr. Jekyll and Mr. Hyde style or even some Q fuckery. Which could’ve actually made it a bit more interesting and helped it fit in with contemporary theories and other storylines regarding multiple universes. So all of the mirror universe episodes ended up coming across a bit like Voyager’s Threshold to me, and making that an integral part of the ongoing plot of Discovery is a large part of what ruined that series for me. That and the similar ludicrousness of the mushroom drive.

Technobabble was basically invented for Star Trek, I dont know where you get the idea that it’s somehow based on some verifiable scientific principles. Its nowhere near (how I hear) shows like The Expanse approach science fiction. 99% of the time they just plain make something up to explain away a plot point.

Heck, the very first episode is “What if certain people become gods if they leave the confines of the galaxy”. I dont know what scientific principles that was based off of, but I’d like me some of that.

As far as the origins of its dark nature, I believe that was (technically) explored in Enterprise. The main difference is that, when the Vulcans landed to make First Contact with humans, the humans decided to violently take over the Vulcan technology instead of peaceful(ish) coexistence. Not exactly a big stretch, and (loosely) based on scientific theories and sci-fi tropes regarding alternate universes.

It became a major part of the entire Discovery tv show.
Ah, I never watched it, at least partly because of that. It took something whimsical and made it melodramatic (from descriptions I’ve read, at least). That seems more an effect of how it was used in that instance rather than the “plot device” itself.
That example only shows how shite Discovery is, that it could taint such a timeless trope as the mirror universe. 🤌🏼
I only responded because of the claim " less than nothing to do with plot". Which is, like it or not, absolutely not true in anyway
Technically accurate is the best kind.

Never let a working man slander the name of my father.

He was the union man.