• The episode title references the mirror universe, a dark reflection of the familiar reality of Star Trek where humans, or Terrans as they’re more commonly called there, evolved to be more sensitive to light, resulting in everyone tending more towards malevolence, and barbarism, and queer coded villainy. Other episodes involving the mirror universe that reference mirrors, include:

    • “MIrror, Mirror”

    • “Through the Looking Glass”

    • “Shattered Mirror”

    • “In a Mirror, Darkly”

    • “In a Mirror, Darkly, Part II”

• Despite not being Starfleet, Book apparently keeps a personal log. He records the stardate as 866282.9.

    • Other non-Starfleet personnel whom we know kept logs include: Neelix, Seven of Nine, and T’Pring.

• The digital ”Federation Watch List” wanted poster for Moll shows the emblems of:

    • Starfleet

    • Ni’Var

    • United Earth

    • Trill

    • Fernginar

    • Risa

    • Hornish

    • Orion

    • Andoria

    • We also see Orion and Andorian files on Moll, including Orion and Andorian script, first seen in “Borderland” and “The Andorian Incident” respectively.

• Rayner suggests to Burnham that the mission into the wormhole is too dangerous for the ship’s captain to take themselves. Picard says it’s a general policy in “Time’s Arrow” that the captain does not join away teams, and in “Star Trek Nemesis” Data sites a specific regulation. However, no captain we’ve seen other than Picard really observes this regulation.

• On the other side of the wormhole, Burnham and Book find the ISS Enterprise. The ship’s only other appearance was in “Mirror, Mirror”. For this episode, the Constitution-class appearance seen in both DIS and SNW is used for the ship, and redressed SNW sets are used for the interior.

    • In “Despite Yourself”, a wireframe model of the Constitution-class USS Defiant was displayed aboard the USS Discovery; at that time the ship had been in Terran Empire custody for over a 100 years, and appeared to have some alterations to both the nacelle pylons, and the bridge, but apparently when the Terrans got around to building their own Constitution-class, they opted for a configuration closer to the original.

• It was established in “Die Trying” that ”Crossing between universes has been impossible for centuries.”

• ”That was my brother’s station, aboard the USS Enterprise*.”* Burnham was raised by Sarek after the apparent death of her parents, as established in “The Vulcan Hello”.

    • ”I’m sure he was just as ruthless as the rest of them.” We learned in “Crossover” that mirror universe Spock became High Chancellor of the Terran Empire, after being inspired by Kirk in “Mirror, Mirror” and instituted major societal reforms, making the Empire more peaceful, resulting in it being conquered and enslaved by the Klingon-Cardassian Alliance.

    • Book finds a plaque with the story of the mirror Enterprise claiming that they escaped to the prime universe after the High Chancellor was killed for attempting to institute reforms. Presumably this still refers to mirror Spock, though he’s not mentioned by name.

    • Burnham and Book assume the ”Kelpien slave turned rebel leader” who helped the mirror Enterprise escape was mirror Saru, whom we saw in “The Wolf Inside”.

• Burnham find a plush doll of a mirror universe Gorn. Mirror Gorn, of course, also abduct members of other species to use as host bodies/food on their breeding planets, but in the Terran Empire that is considered to be a cuddly trait.

• Moll and L’ak created multiple holographic duplicates of themselves to stymie Book and Burnham. The Doctor did something similar in “Renaissance Man” by filling the holodock with copies of himself to escape Tuvok.

• We learn that L’ak is a Breen, a species whom we the audience have not previously seen outside of their refrigeration suits.

    • In “‘Til Death Do Us Part” Worf claimed that no one had seen a Breen outside their suits and lived. Though in “Indiscretion”, three seasons earlier, Kira and Dukat did incapacitate some Breen and steal their uniforms to use as disguises, so Worf’s claims are about as accurate as usual.

• In flashback we see a station operated and populated by Breen. Though their helmets no longer resemble something a character might wear during a War in the Stars, the asymmetric design of their refrigeration suits is inspired by what we saw in DS9.

• We learn through the flashbacks that Moll was saving latinum to be able to afford to set herself up on a colony in the gamma quadrant that she had never been to, but was described to her by Cleveland Booker as being the perfect home. In the season four episode, “The Galactic Barrier” we saw Tarka’s flashbacks to his developing a relationship with Oros, and their mutual obsession with finding a way to an alternate universe that was supposed to be a paradise.

• Unlike what we’ve seen of the Enterprise in DIS and SNW, it’s mirror counterpart has been upgraded with the same system aboard the USS Discovery A that belches gouts of fire into the bridge whenever it encounters a bit of turbulence.

• During a scuffle with Burnham, L’ak ends up stabbing himself, an advanced fighting technique usually only attempted by the most feared Klingon warriors, such as Kozak in “The House of Quark”, the Torchbearer in “The Vulcan Hello”, and most recently Dak’Rah in “Under the Cloak of War”. L’ak has not quite yet mastered the move though, as he lived.

• We learn that L’ak “Carries the genetic code of the Yod-Thot. Those that rule.” In DS9, Thot was a rank held by Breen flag officers.

• Book asks Burnham if she wants to give Pike’s catchphrase, “Hit it,” but she declines. Presumably Book looked up the catchphrases used by various captains of the Enterprise at some point.

• Detmer and Owosekun get to head a team to fly the mirror Enterprise back to Federation HQ. Rhys, whom it has been established twice this season in “Jinaal” and “Face the Strange” loves the Constitution-class more than any other ship, punches a bulkhead when he hears the news.

• A gormagander is a colloquial referred to as a space whale, and they were introduced in “Magic to Make the Sanest Man Go Mad”.

• Doctor Culber mentions having died, which he did when Ash Tyler snapped his neck in “Despite Yourself”, his resurrection in “Saints of Imperfection”, and then hosting the Jinaal personality in “Jinaal”.

• The episode was dedicated to Allan “Red” Marceta, a set dresser who passed away in 2022.