The dictionary definition of "rubicon" is "a limit or point that is reached when the results of one's actions cannot be changed." As we set up for the endgame of Season 4, this episode marks the point where there is no going back, for Book, Tarka, Michael or anyone else. Tarka and Booker build their weapon to destroy the Dark Matter Anomaly, and end up setting it off despite Michael and her crew's best efforts. The results are not what anyone expected, the egotistical asshole Tarka least of all. (It's a measure of the actor's strength in the role that you feel a bit of sympathy for him, even as you're yelling at Book to punch him out.)
In between, Michael and Book play a rather delicious game of cat and mouse, each trying to stop and/or warn off the other without seriously injuring anyone. Those two know each other so well, and fight using a gambit employed during their courier days. (In particular, you can see that David Ajala as Book is enjoying every minute of this.) The final shot of the two of them, when Michael offers her Federation-approved compromise that Book accepts--until Tarka ruins it--showcases a mature relationship with the people involved unfortunately ending up on opposite sides for completely understandable reasons. But Michael trusts in Book being reasonable, and Book trusts in Michael being truthful. It should have worked, and would have worked, were it not for Tarka throwing his last desperate monkey wrench into the middle of the whole thing and blowing it up.
(And all for an egotistical assumption he shouldn't have made, that the DMA also carried within itself its power source, and once the mechanism that controlled it was shut down, the power source would have been laid bare for anyone to find. Unfortunately, we find out the power source is on the other side of the DMA's wormhole, outside the galaxy and the energy barrier surrounding it. Which the supposedly "genius" Tarka never considered? Apparently not, as he is stunned. Even worse, shortly after the first anomaly is destroyed, it is replaced with another one, as casually as you or I would replace a lightbulb.)
The theme for this episode is "finding a middle ground"--between Michael and Book, Michael and the Federation, and between Michael and the woman assigned to monitor her during this mission: our old friend Commander Nhan, the security chief who rather abruptly departed Discovery, and the show, last season. (But Michael and Nhan had, and still have, a warm relationship, which was heartening to see.) Nhan pushes Michael to the brink during the tense cat-and-mouse exchange, presenting her with a last-resort option (a program that exposes a flaw in the prototype spore drive Tarka stole and installed in Book's ship, which will destroy the ship if used). Nhan makes it clear she is fully prepared to issue the order if necessary, and Michael accedes to her authority. But she also comes up with a last-ditch solution of her own: she asks Stamets and Zora to calculate the rate the DMA is mining boronite in the sector, and determines it will be a full week before it depletes the local supply and moves on. Armed with this knowledge, she asks Nhan to let her propose her compromise to Book: if they stand down for a week and allow the Federation to attempt first contact, and by the end of the week Species 10-C does not agree to withdraw, Tarka's original plan to destroy the DMA will be approved.
This is all talked out as professionals and adults, aside from a few moments of stressed tempers flaring, which the ever-calm Saru defuses. (There is, however, an unfortunately clunky plot hole in all of this: that everyone virtually ignores Tarka, despite Dr. Culber rightly labeling him a "wild card." Of course this bites everyone in the ass at the end. I really hope somewhere along the way, somebody gets to punch Tarka's smug face.) At the end, Commander Nhan even admits things are not always so binary, so black and white, and says she will seek some "middle ground" of her own.
Saru also gets two delightful little scenes in this episode, which serve to further his burgeoning relationship with Ni'Var President T'Rina. Troubled by everything he is dealing with, Saru contacts T'Rina through the miracle of long-distance hand-wavey subspace phone calls and asks her to help him with his meditation. T'Rina says she often finds the simple routine of everyday life helps settle her mind, and offers to join Saru for a meal. After everything that happens in the episode is over, Saru goes to discuss this with Dr. Culber:
"Considering what we're dealing with, I feel silly for bringing this up, but I'm in need of advice on how best to respond to an overture from President T'Rina."
"She asked me to join her for dinner. Socially. Of course, I cannot, given the mission at hand, but I'm unsure how best to convey that even if it were possible I would decline. She and I hail from vastly different cultures [and species, for that matter]. I already feel pulled with my commitments to Kaminar and Starfleet."
Culber is barely managing to smother his grin through all of this: "Do you have feelings for her?"
"I think," Saru says, tilting his head in sudden realization, "perhaps I do?"
"With respect, sir, in my professional opinion, you're being an idiot. Connection is always a risk, but compared to everything else we're dealing with right now, I'd say it's one that you can handle."
This is acted to perfection by Doug Jones and Wilson Cruz.
So now that Tarka has blown everybody's best laid plans to smithereens, Michael must prepare for a first contact with a species that replaces planet-destroying mining equipment like the flick of a switch. This is not the best episode of the season (that honor still goes to "But To Connect") but it was an engrossing hour that sets up well for however many episodes remain in the season.