It's the first two-part episode for a while, and... well, I enjoyed it up until Georgiou went to the Mirror Universe. After that, my god, did it become boring.
The problem with the Mirror Universe is that it's an idea that becomes stale quickly. Discovery season one just about managed it for an extended run of episodes, but then we had the primary crew to maintain contrast. This time, we're spending time with nothing but a bunch of power-hungry murderers, and it's tedious. Enterprise got away with it with "In a Mirror, Darkly," because it had at least some humour, we knew the equivalent characters well, and the chance to learn about the history of the Mirror Universe was interesting. And they through some classic aliens in, which were the main draw for many of us. On Deep Space Nine, the Mirror Universe episodes were funny, we only got one a year, and even then they started to wear thing by the seventh season.
In "Terra Firma," we spend the better part of two hours in the company of an utterly reprehensible woman, who we're supposed to feel sympathy for, and for all of Michelle Yeoh's acting skills (and she really is excellent), it's just very hard to care about what happens to her. So this mass-murdering monster is going to vapourise? Fine, good riddance. She's been a tedious character anyway until this season's improvement in her writing, but it's not enough to actually care about her. Villainous characters on a road to redemption is a well-run road, and it can work very well. In recent years, redemption arcs for villains, especially female villains, have been a big thing - Missy on Doctor Who, about 90% of Once Upon a Time - but you have to have enough charisma to actually want the villain to succeed. You have to want to see her do better. I just don't care about Mirror Georgiou enough.
I can see why Georgiou and Burnham care for each other; they remind each other of the person they lost through a betrayal and it gives them a way to make amends, through which they have come to genuinely care about each other. But why the hell do any of the rest of them give a damn about Empress of Evil? Tilly even gives her a hug, for crying out loud. And as for the officer's mess toast for the worst person any of them have ever met... you can't convince us that either the characters or viewers care about her by having a few speeches and a toast afterwards. I get that, past the fourth wall, they're sending off Yeoh, but the whole point of this endeavour was that she'd get her own series, so we'll see her again in a year or two, once she's finished with Shang-Chi.
It begins well. Georgiou's affliction is genuinely interesting, and having Kovich (aka Cronenberg, don't just call him Cronenberg, remember his name) explain the effect via a fan-pleasing bit of exposition is fun.Added to this is the ongoing mystery of the Burn, which is finally heading towards some kind of resolution thanks to the source of the distress call at its origin being identified. The ship is Kelpien, which potentially adds a whole other layer of intrigue and divided loyalties for Saru... or might amount to nothing. We don't know yet. Vance inexplicably allows his most valuable ship to go off on a jaunt into the unknown to help a genocidal dictator from a parallel universe, so we get to visit another planet thousands of light years away, where Burnham and Georgiou are greeted by the mysterious Carl. (You think a cosmic entity called Carl is silly? The Kelpien ship seems to be called Keith, so whatever. Prosaic names are in in the 32nd century.)
Carl is fabulous. Paul Guilfoyle makes him both mysterious and likeable. He's an all-powerful celestial being who smokes a cigar and loves puns (really bad ones - our senses of humour are identical). He's very Doctorish, really, and the immediate question is, what or who is he? Is he a Q? A Prophet? The Guardian of Forever? Well, the clues were there for that last one, not least the issue of The Star Dispatch newspaper he was reading, complete with an Edith Keeler-related headline, but there were several other Easter eggs for the eagle-eyed, so it could all have been a red herring. I had kind of hoped he'd be something new - Discovery has been trading very heavily on established alien races lately - but making him the Guardian works. It's fun, it's a nice callback, and why shouldn't the Guardian have evolved during a millennium of dealing with humans? It also calls back to Harlan Ellison's original intention that the Guardian was one of a race of powerful beings, not the portal itself. Here, he's both!
Sadly, it's all to facilitate that interminable trip to the Mirror Universe, where everyone scowls and wears bad emo make-up and hams it up atrociously. Sure, the cast all look like they're having fun, but it's so tiresome. It almost works with Mirror Burnham, Captain Killy, etc. but they're characters are too broadly sketched. They're just bad. That's all there really is to them. The minor counterparts could have worked, but we don't know enough about the prime versions of Detmer, Rhys, Owesekun et al for their evil opposites to mean anything. We just get a long sequence of nasty soldier types strutting about and being violent. All to show how Georgiou has apparently turned over a new leaf. Yeah, her hands were tied when she killed Burnham - it was life or death - but that's only after torturing her for days. The scenes with the apparently nameless Mirror equivalent of Saru are better, and it will be interesting to see if there are real repercussions for the Kelpiens in that universe (I had presumed Georgious was planning to make them into a secret army, but that's not going to happen now). Otherwise, though, it's not enough to show that Georgiou has improved from what she was before. She's on the way, yes, but not enough to be let loose on the universe.
Ultimately, Prime Burnham had the best version of this story: she skipped the entire Mirror Universe sequence and saw Georgiou just pass through a door. At least the uniforms looked smart.
Dannus V (or Dennis V as I kept hearing it) is located at the galactic rim, on the cusp of the Gamma Quadrant.
Kepler-174d, where Mirror Burnham committed some colourful atrocities, is a real planet. With a mass 5.4 times that of the Earth and roughly twice the radius, it's considered a mini-Neptune-like planet. Its the outerrmost of three known planets orbiting a star 1269 light years from Earth, in the constellation of Lyra.
The main shipyard for Mirror Starfleet orbits Epsilon Indi II, a mere twelve light years away. It has one know jovian planet and two brown dwarf companions, any one of which might be considered Epsilon Indi II under Trek nomenclature.
Risa in the Mirror Universe looks very different to its Prime Universe counterpart. It even has a ring system. Prime Risa has two moons, so I'm guessing they were destroyed in a battle against the empire and what's left of them makes up the rings.
Timeline Data: Part one features the first direct confirmation of the Kelvin timeline in televised Trek, by way of a Betelguesian time soldier named Kor who originated in 2379 of that reality. Kovich refers directly to the reality being created by a Romulan temporal incursion. Being seperated from both his native time and reality destabilised Kor's matter and he disintegrated, the same thing that's happening to Georgiou. Neither Spock nor Nero and his crew suffered this in the 2009 Star Trek but that was a much smaller jump in time. The Romulans survived twenty-five years in that reality, but they did create it, so perhaps that made a difference. Spock died after five years, but he was 161 years old.
The Interdimensional Displacement Restriction was part of the Temporal Accords, and banned travel between realities. Except, again, there will be people from realities where it isn't banned who use it and could cross over to the prime timeline happily.
It's unclear if Carl actually sent Georgiou into her own past or merely created a false timeline to test her. If the former, then presumably the history of the Mirror Universe has changed, or at least, a divergent timeline has been created. It's at least hinted that the latter is the case, but it's not quite clear.
It's interesting that Georgiou refers to the entire empire as Terra, not just the Mirror Earth. But why is the rallying cry for the empire "Terra Firma?" That just means "solid ground."