This episode is something of a rarity in Star Trek nowadays: a self-contained bottle episode, with only one guest star (or two, I guess, if you count the voice performance of Annabelle Wallis as Zora, Discovery's computer). It also has a noticeable absence of shaky-cam pew-pew, except for one shot when the ship is trying to mushroom-jump and can't and the camera spins wildly on the bridge, reflecting the fact that Discovery is whirling madly in space and getting nowhere, before slowing to a halt. (However, they still have the annoying "sparkler" effects, blue sparks cascading down from the ceiling of the various sets to indicate damage.)
This episode also moves us just a little closer to the Short Trek filmed three years ago, "Calypso," where we drop in on an abandoned Discovery and the ship's computer, all alone, bonding with an unexpected passenger. In the last episode, Zora (as the sentient combination of computer/sphere data has named herself) admitted she had developed emotions. In this episode, her newfound emotions get a workout: she is so overwhelmed by all the internal/external sensory inputs that she cannot focus, and she is so afraid of the situation she and the crew are in and what she has to do to get them out of it that she comes near to freezing up and not doing anything. Only some gentle, perceptive talking by both Gray (making good use of the character now that he's got his synth body and can have his own storyline, separate from Adira) and Burnham convince Zora to acknowledge her emotions, work through them, and do what needs to be done.
Zora's emotional crisis would not matter if not for the fact that Discovery is ordered to investigate the subspace rift caused when the Dark Matter Anomaly abruptly shifted location last episode. When Burnham takes them past the plasma barrier and inside, there is nothing. No stars, no normal background radiation and various other electromagnetic noises stars, nebulas and planets make all across the spectrum--it's a void almost like being inside a black hole. Except that it's a helluva lot creepier, as the crew finds out when they send out a DOT to investigate, and the poor thing gets 6000 meters away and is slowly fragmented to nothingness. As Adira notes, it made a noise like it was screaming as it died. Whatever killed the DOT is slowly advancing on Discovery, and they cannot back out of the rift as they are lacking their normal fixed galactic points of reference. Burnham orders Stamets to jump them out, and Book (who has gone down to engineering asking to help) tries to navigate them through the mycelial network, only to find it has been damaged by the rift and they can't use that to escape either.
So the episode comes down to a race against time trying to figure a way out before the void closes on them, eats away at the hull, and destroys the ship. Along the way there are a couple of subplots: as noted, Gray starts talking to Zora as he is sitting in the lounge waiting to see why the captain ordered the red alert after they entered the rift. He notes how nervous and emotionally wobbly she is, and starts playing a Trill game with her, the same game Trill Guardians use to calm new hosts. This distracts Zora and helps her focus. The other subplot is Book's dealing with the memory (or maybe spirit) of his dead father that he sees after he tries to jump Discovery out of the rift and is shut down by an energy surge. This brings on hallucinations, and Book has a few scenes where he argues with his father, mainly over Burnham. I regard this as foreshadowing that their relationship is going to hit a rough patch later in the season, as Book's father insists that Burnham "will pick Starfleet over you and Kwejian every time," reflecting what I think are Book's own unspoken doubts. What happens to Book also supplies the major reveal in the episode, because the particles left in his brain following the energy surge come from one place, the energy barrier surrounding the galaxy. The DMA has an extragalactic origin. This also provides the way out for Discovery, as they find they can use echolocation, "pinging" those particles at a specific frequency to find where they were deposited at the entrance to the rift, and Zora can follow the echo to take the ship out.
Needless to say, it's not as easy as all that, because the rift is closing in and the shields will collapse before they can exit. Burnham comes up with the idea of storing all the crew in the transporter pattern buffer, shielding them from the heat and hull breaches until the ship can exit the rift. Of course, this sets up another only-Burnham-can-save-the-day scenario, as she stays behind on the bridge in an EVA suit. In this case, however, it's less Burnham and more Zora, as Burnham talks to the ship's computer to calm and distract her, and as the temperature rises and Burnham needs her own distraction (she's starting to cook inside her suit), Zora sings the classic jazz song "Stormy Weather" to her. Because Burnham passes out, Zora has to finish bringing the ship out of the rift and release the crew on her own.
At the end, as Discovery is back at the Archer spacedock undergoing repairs, there are a couple of nice scenes. In the first, Saru and Book talk about anger and how to handle it. Book says he is afraid of being angry at the world just like his father: "My father had so much anger in him. I told myself I'd never be like that. Maybe I am. All I want to do is destroy them [whoever built the DMA]."
Saru: "I understand. The Ba'ul culled my people for centuries. My parents died at their hands. Now I sit across from them at the Kaminar council. I still feel rage."
"But you seem so balanced. So calm."
"We are both justified in our anger. Allowing it to be our focus, however, only prevents us from achieving those things which serve the greater good. It is a struggle, yes. But a worthy one."
This is a small scene, perhaps, but it is masterfully acted by Doug Jones and David Ajala.
The closing scene is of Burnham borrowing last episode's lilagio orb and creating her own holographic family tree, with images of Spock, Sarek, Amanda, Gabrielle, Emperor Georgiou, and others. Zora asks if she can create one, and she does: far larger, adorned with holos of all the Discovery crew members.
This episode was directed by Jonathan Frakes, and had some nice scenes of the bridge crew working together to solve the problem (and we get to see Detmer and Owosekun again, and actually find out a little more about the latter!). It also had good uses of other characters, particularly Zora and Gray. It's definitely a quieter episode, but well-paced and intense. I liked it.