This episode definitely advances the plot....but there aren't any action scenes. Instead, there are epiphanies and conversations, finding out what happened to past characters and more conversations, and a weaving of elements from every Star Trek era and more conversations. It sounds deadly dull, but damn, it works. This has supplanted "Forget Me Not" as my favorite episode of the season.
There are three storylines here. The first is Book and Michael Burnham getting together. His (still nameless) ship is still sitting in Discovery's shuttle bay, and since this picks up right after the previous episode where Michael was disciplined and removed from her position as First Officer, she ends up running to Book. The voiceover tells us (as if we didn't already know) that her year in the future has changed her, and she's no longer sure if she fits on Discovery or if she should stay there. After she and Book do a little mattress dancing, she admits that to him. He asks her to leave the ship and go away with him, and presumably they would take up where they left off before Discovery arrived, living as couriers in this strange, fragmented new world.
Of course, this is not what happens. There's a reason Sonequa Martin-Green is first in the credits, and naturally Michael works through her issues and decides to remain on the ship. No matter that, strictly from the viewpoint of her character, it might be better for her to live the wild maverick life free of Starfleet restrictions, even upside-down future ones. Not to mention giving the other characters the room to shine they sorely need. (If Martin-Green ever left the show, it could very much carry on without her. That's a testament to the impeccable cast and the way they have been written, especially this season.) But she opts to stay, and now it's up to Book to decide what he wants to do. That is still up in the air at episode's end.
The second storyline also draws from the events of the previous, namely Michael's demotion. Saru summons Tilly to his office and offers her the position of acting First Officer. Now, this was not really much of a surprise to me, as it's been obvious in previous episodes that the two of them have established a rapport that was not there before. Tilly feels she can speak freely to Saru, and he seems to value her advice. She points out to him that there are many other people on the ship qualified to step into the position, and she never even finished her command track training. Saru replies that in the situation they're now in, that doesn't really matter, and she has comported herself admirably since their leap to the future. Tilly asks if he's picking her because she's qualified or because she's compliant--a fair question, and one Saru neatly sidesteps. He suggests she take a day to think about it, and she immediately goes to Stamets and asks his opinion. Stamets does note how weird it would feel to take orders from her, but you can see he doesn't have any objection to it. This is reinforced towards the end of the episode when Tilly comes down to the engine room to find the entire bridge crew there, urging her to accept the position. Even Michael, catching up with the group after the episode's other events, tells her to say yes, and she finally does.
This has already led to blowback from some fans, saying that any of the bridge crew would have been more qualified. This is true, but the problem is that the show has written itself into a bit of a corner--by focusing so much on Michael, we don't know enough about most of the other characters (especially the bridge crew, hence my unceasing drumbeat of Give Them More To Do! Tell Us More About Them! Even one scene or B-plot storyline per episode, focusing on a different bridge crew member, would suffice!) to see if they could handle the job or not. (And the other character I would have loved to see as Saru's Number One, Tig Notaro's Jett Reno, isn't a regular. That should also be rectified, immediately.) Since Michael's been shuffled back to chief science officer, Tilly is kind of the only one left. Which is admittedly not optimal, but it will be interesting to see what the writers do with her now.
The main storyline, however, deals with Michael's obsession with finding out the cause of the Burn. After downloading the data from Book's "black box," she and Tilly discover there was a definite lag in the timelines of the starships exploding, albeit in millionths of a microsecond. The Burn did have a point of origin, but the section of space it would be within is still too vast to search in several lifetimes. However, there is a series of sensor relays from an experiment called SB-19 that could have more data. But when Michael and Saru go to Admiral Vance and ask for the data from SB-19, he tells them he doesn't have it. It belongs to the planet Ni'Var; or as he says, "You'd know it by its former name, Vulcan."
And therein comes our episode's title. Centuries after Ambassador Spock's quest to reunify the Vulcans and Romulans, as depicted in The Next Generation episode "Unification II," his efforts have succeeded. Vulcans and Romulans are living together on the planet. Unfortunately, they have also withdrawn from the Federation, more than a century earlier. As the President of Ni'Var, T'Rina, later informs Saru, the tensions between the planet and the Federation were already present: the Federation was too large, spreading itself too thin, and dilithium was running out long before the Burn. SB-19 was Ni'Var's attempt to come up with an alternative to dilithium-based warp travel, and the Federation pushed them to continue with the experiment even though they didn't feel it was safe. The Burn happened shortly afterward, and they are convinced it was their fault. Michael says someone has to talk to them, convince them to turn over the data--and Admiral Vance (of course) comes up with sending the sister of Ambassador Spock, the Unifier, to Ni'Var to do just that.
(There is a lovely scene after this revelation, when Michael returns to Book's ship and delves into Starfleet archives chronicling the life of her little brother, something she has not dared to do before. We see an overlay of Leonard Nimoy himself, in a snippet of dialogue from "Unification II." Michael is overjoyed, and you can see it in every line of her face. I might wish the show gave a bit more attention to the other characters, but Sonequa Martin-Green does have some serious acting chops.)
The Discovery jumps to Ni'var, and T'Rina, sends her hologram to the bridge. She is pleased to see them and greets the sister of Ambassador Spock warmly, but promptly turns down Michael's request. The "delicate balance of reunification" is still an issue. Michael, of course, blows past this, asking if the people of Ni'Var still "hold to the old ways" and then springing her unwelcome, apparently spur of the moment surprise on T'Rina: "As a graduate of the Vulcan Science Acadamy, I invoke the T'kal'in'ket." T'Rina cannot turn this down, as much as she would obviously like to, and says she will prepare the Quorum.
(And after her hologram flickers out, there is a delightful moment when Saru turns to Michael and says one word: "Commander?" with the pitch-perfect inflection of "WHAT SHIT HAVE YOU DRAGGED US INTO NOW!?" It's amazing how much Doug Jones can convey with just his voice, or in the case of his starring turn as the ancient god in The Shape of Water, no dialogue at all.)
The T'kal'in ket is the demanding Vulcan ritual of defending one's hypothesis before three judges: in this case, one Romulan, one Vulcan, and one hybrid, representing the three uneasy factions of the planet. One difference from Michael's day: she gets an advocate, a member of the Qowat Milat, the Romulan warrior-nun sect devoted to "absolute candor" who bind themselves to lost causes (this is the callback to Star Trek Picard). When T'Rina and the Quorum come on board, she tells Michael her Qowat Milat advocate has taken a "particular interest" in her case. Michael meets the sister assigned to her, the woman throws back her veil--and it's Gabrielle Burnham, Michael's mother. After she was flung back into the future in Season 2, she went to Ni'Var hunting for Michael, and fell in with the Qowat Milat. While they walk to the room where the ritual is to be held, they catch each other up, and Michael makes several confessions to her mother: mainly, that she is not at all sure if she fits into Starfleet anymore. Which is understandable, but still, she should have remembered who she was speaking to.
The ritual begins, and it does not go well. The Vulcan Quorum member, who is a condescending, snotty logic extremist, rejects Michael's assertion that she has discovered that Ni'Var did not cause the Burn. All three Quorum members say they do not trust the Federation with the SB-19 data. Michael attempts to present herself as trustworthy, not being manipulated by Starfleet. Neither assertion is quite true, and Gabrielle knows it. Finally, she takes things in hand and lays Michael's character and motives open, revealing all her flaws and bad decisions over the course of two seasons of the show. This forces Michael to confront all her equivocations and doubts, and the viewer can see her working through the things she has done in real time, the sacrifices she and the crew of Discovery have made that got them to this point. She admits she is afraid of losing everything she has paid so dearly for. Which of course doesn't have anything to do with what she's there for, but it allows the Quorum members (and other observers, which as Gabrielle said are the more important ones) to see her stripped bare of all artifice. The Quorum members start fighting over her request, and Michael walks over to the gong, hits it, and says she is withdrawing it. If granting her request would destroy the peace of Ni'Var, she will find another way. She then walks off. After a bit, Gabrielle finds her in another chamber, and sort-of apologizes for what she did. She tells her daughter that she doesn't have to choose between the person she is now and the person she used to be, and reveals that by opening herself up the way she did, she convinced one observer--President T'Rina--to give her the data. Gabrielle is also going to stay on Ni'Var with the Qowat Milat.
So in the end, Michael Burnham gets what she came for and works through her issues at the same time. This weaving of several generations of Star Trek lore was expertly done, and I liked the fact that there wasn't a phaser or a shootout to be found. Not that one can't have some good action scenes too (particularly when Emperor Georgiou is around), but settling things by talking is, or should be, the Star Trek way. Anyway, like I said, I really liked how this played out. It was well-written, structured and paced, and is a good example of how Discovery has raised the bar this season.