"Forget Me Not" is a refreshingly, unusually quiet and introspective hour of Discovery that feels like it's from a previous generation of Star Trek. It's among the quietest and most character-driven episodes of the series. It's perhaps not the most riveting episode of all time and it employs plenty of mystical alien mumbo-jumbo, but it's very nicely done and sincere, and it benefits from a final revelation that's truly intriguing — building on a concept previously explored by adding just a slight little spin on things that's rather ingenious.
The episode takes us to the Trill homeworld, to see if they can help Adira — the only known successful example of a human joined with a Trill symbiont (I guess Riker in "The Host" doesn't count because it was temporary) — recover the Tal symbiont's memories, which are blocked from her. (These memories include those of Admiral Senna Tal, who may know where to find what remains of the Federation.) Indeed, Adira can't remember how she even came to be joined with the symbiont in the first place.
After engaging in a few Trekkian standbys — (1) the Trill homeworld is reduced to a couple rural locations and caves on the outskirts of society and a cast of a half dozen guest actors; and (2) the initial conflict between our characters and the Trill (Adira is an abomination for having been joined with a human!) feels forced and unnecessary — the more sympathetic Trill help Adira by taking her to a Spiritual Sci-Fi Cave (presumably the same one seen in DS9's "Equilibrium") so she can break down the barriers and reclaim the memories of the symbiont. Burnham tags along to provide moral support for the new character amid the trials and tribulations.
Even as a quieter outing, this maintains a high VFX game by making the Trill caves and Adira's internal experience much more visually elaborate and distinctive than what "Equilibrium" could muster a quarter-century ago.
The B-plot is also an introspective affair that wisely examines how the crew is quietly suffering from isolation while trying to get on with regular business. (In a nice example of starship operational prudence, Saru recommends looking for a solution to use the spore drive that's not dependent solely on Stamets' ability to interface with it.)
Discovery is an island unto itself, and even if that was always the case as long as the ship was at war or on a mission, it's different now as the crew realizes that's forever, which eats away at their psyche. The story makes smart use of Dr. Culber by having him provide Saru with a status of the ship's mental health. This leads Saru to pull together a morale-building dinner for the bridge crew, which starts out mildly awkwardly and goes quickly downhill from there. (Detmer's quietly simmering PTSD climaxes with a macabre haiku envisioning the bloodletting of Stamets, whom she verbally attacks as self-important.)
It's nice to see Saru trying to connect with the crew and spurring them to connect with each other, even if the way things deteriorate in the dinner scene, and then are easily patched up at the end of the episode with the movie in the shuttle bay, are a bit pat. But this is solid and necessary material to make this ship and crew feel real, and it's the sort of thing that has often been missing from this series. We also get the resurfacing of the sphere data that's fused with Discovery's computer, which helps Saru by providing ideas for how to make these personal connections. The idea that this strange AI might actually become a functioning part of the ship's community, as opposed to a dangerous ticking time bomb, provides some hope. The crew could use some help out here. It's lonely.
As for how Adira received the Tal symbiont, it turns out her boyfriend Gray (Ian Alexander) was a Trill who received the Tal symbiont while they were in a relationship. It changed him and their relationship, but they were adjusting, and the new memories provided new gifts. (My gut reaction is that Adira and Gray are far too young to give this relationship quite the weight that it needs, but I suppose joining Gray with a symbiont would make him wise beyond his years.) But then Gray was mortally wounded in a catastrophic accident (an asteroid hits the ship; where were the shields, who was in charge?!), and the symbiont was transferred to Adira in an emergency to save it.
This has intriguing implications in how it reimagines the concept of loss and remembrance through sci-fi invention (and asking human questions through sci-fi what-ifs is what true sci-fi is all about, after all). It does this using familiar Trill concepts explored in DS9 episodes like "Rejoined" (and even TNG's "The Host"), but puts the added twist on things by having the symbiont joined back with the deceased host's lover, who now holds all of his memories. In "Rejoined," resuming a relationship from a former host's life was strictly forbidden by Trill society; they probably didn't even have a rule for what happens here.
When a loved one dies, the love that once existed between two people now exists only within the survivor's memories. With Adira Tal, now directly connected to all the symbiont's memories, we see something unusual has happened. Gray appears to her almost as if he's actually there, and they can have a conversation, like talking with a ghost. So odd that must be. (Comforting? Painful? A distressing form of multiple-personality disorder?) Is it a blessing or a curse? Part of grief is moving forward, but Adira will never be able to. Or maybe the way to look at it is that she will never have to.
Let's talk about the casting. Gray is played by Ian Alexander, who identifies as transgender male, and Adira is played by Blu del Barrio, who identifies as non-binary (and uses they/them pronouns). It seems especially apropos to cast these actors in these particular Trill roles considering a Trill symbiont lives many lives as different genders and the individual currently carrying the symbiont experiences the memories of all of them. One looks back at "Rejoined" from 25 years ago and can see how it used LGBT+ themes, and one looks at "Forget Me Not" today and sees it approached from yet a different LGBT+ angle. (From the pronoun standpoint, Adira has been referred to as "she" even though that's not how the actor playing her refers to themself. But now, with this melding of Trill personalities, that may change, except with "they" taking on the conventional meaning of multiple people.)
Maybe I'm giving too much credit to the writers here, but this casting is food for thought both inside and outside the text. Adira's specific storyline is a reference to — although not with any direct allegorical parallel — how we're increasingly societally more aware of non-binary identities. This isn't like TOS trying to sneak things past the censors in the 1960s (Discovery's writers aren't hiding what they're doing), but it still speaks to underrepresented groups getting some focus in a way that serves a specific story. Intellectually speaking, that's interesting. I've probably written more about the sci-fi substance of "Forget Me Not" than any episode of Discovery to date. That's saying something.