Burnham and Book follow Moll and L'ak into a pocket dimension in search of the next clue.
Tropes:
- Affectionate Nickname: Moll dubs L'ak "green eye" in flashbacks, based on the design of Breen helmets.
- Arc Welding: Some of the longstanding TNG-era mysteries about the Breen Confederacy and the Mirror Universe Myth Arc (both from Burnham and Kirk's cross overs) get tied into the ongoing Progenitor Technology Hunt.
- Art Evolution:
- The Breen armor and helmets have been redesigned and updated from their DS9-era template, removing the prominent nose-like protrusion but keeping the green visor line (both to account for the Time Skip since the 24th Century and also likely to less resemble Boussh from Return of the Jedi this time around).
- The I.S.S. Enterprise returns from the first time since "Mirror, Mirror"... but uses Star Trek: Strange New Worlds's sets and Constitution-class design. No one acknowledges the differences in visual design, suggesting the new designs are meant to be a retcon.
- Back for the Finale: In a roundabout sense, the Mirror Universe. Given how important the franchise's most famous and iconic alternate reality has been to the overarching series narrative (and given it hasn't been touched on since Empress Georgiou left in Season Three), DSC's final season revisits the Mirror Universe one more time (albeit in the form of the I.S.S. Enterprise and its legacy).
- Black Sheep: L'ak is actually a member of Breen royalty, but when he met Moll he was relegated to more menial tasks and considered a disappointment.
- Bizarre Alien Biology: Breen are green humanoids capable of surviving in multiple environments. They also have two faces: a solid state one and a fluid state that the Breen think of as their true face. While L'ak argues that both are part of their true form, being half of their nature, Primarch Ruhn claims that the Breen have evolved past their more solid state and that taking on that form just makes them weak. In hindsight, this explains why the Female Changeling was more comfortable around them than other "Solids", as they share a similar nature.
- Borrowed Catchphrase: When Burnham and Book take control of the Enterprise, Book borrows Pike's "Hit it" catchphrase when Burham sits in the captain's chair. Though briefly amused, she decides to stick to her own catchphrase.
- Bottle Episode: The episode requires only one new set: the cargo bay in which Moll and L'ak have their flashback romance. Everything else takes place on existing sets. Slightly zigzagged in that they all take place on existing sets for a different show: the bridge, corridors and Sickbay of Star Trek: Strange New Worlds are redressed, as most of the action occurs on a Constitution-class ship.
- Call-Back:
- Moll’s mother died while mining for rubindium on Callor V, a substance first mentioned in TOS "Patterns of Force".
- Tilly says her long day makes her feel like she has been through a Gormangander's digestive tract.
- Character Development: Over the last few episodes, Rayner seemed to oppose collaboration among the bridge officers in favor of decisive action by the captain. This time, he encourages the crew to collaborate an idea to save Burnham and Book when time is of the essence and the initial plan is no longer viable.
- Chekhov's Gun: Burnham quotes a line from a Kelleran story to Rayner, showing she researched his culture just as he did with human customs. When she needs to send a message through the wormhole without comms, she remodulates Enterprise's tractor beam so it sends out a repeating pattern, which he recognizes from the story and through that context and the nature of the tractor beam deduces what she wants him to do.
- Collapsible Helmet: Thanks to future tech, Breen helmets can now simply retract in whole or in part as needed.
- A Day in the Limelight: For the Breen Confederacy (or rather, the Breen Imperium as they are known by the 32nd century), as L'ak and Moll's flashbacks provide long-needed world-building for the Breen (such as more details about their government and finally answering what they look like beneath their helmets).
- Dead Alternate Counterpart: With the reveal that the Mirror Enterprise is still intact, it means she managed to survive while the Prime version was destroyed over the Genesis Planet.
- Doppleganger Spin: Moll and L'ak rig a holoemitter in Sickbay to make it appear as if there are ten of them in various positions, so Burnham and Book won't know which ones to shoot. The two instead target the emitter, evening the odds to a stalemate.
- Dramatic Irony: Burnham assuming Spock's Mirror Universe counterpart was as ruthless as everyone else from that alternate reality. Only the audience of course knows that Mirror Spock actually tried to peacefully reform the Terran Empire (and that his good intentions backfired catastrophically when the Klingon-Cardassian Alliance smelled weakness and conquered the Terrans).
- Enemy Mine: Book and Moll have to work together to free Burnham and L'ak when the latter are trapped in Sickbay.
- Fakin' MacGuffin: Burnham grabs a random locket and slips a piece of her badge into it, so it'll read on scanners as being from their universe. She then claims to Moll and L'ak, who have located the real clue through that same method, that they have fallen for a trick. A firefight breaks out before the gambit can play out, but L'ak later congratulates her on the attempted bluff after she steals the real clue from him during their fight.
- Fantastic Racism: On discovering his nephew's romantic relationship with a human, the Breen Primarch angrily accuses him of consorting with "lesser beings".
- Flashback: Several scenes show how Moll met L'ak and the two began a relationship in secret, with L'ak eventually rebelling against his kind when the two were caught.
- Going to Give It More Energy:
- To take out the lockdown forcefields, Moll creates a power surge. This, among other things, sends the Enterprise careening towards the wormhole aperture that is much smaller than she is.
- To solve the problem of Burnham about to fly an entire starship through a wormhole barely the size of the shuttle, the crew brainstorms until Rhys hits on the idea of feeding even more anti-matter into the reaction that causes the aperture to dilate periodically. This will stabilize the wormhole and force it open wide enough for the Enterprise to fly through, but only for one minute, after which it will permanently collapse.
- Hard Light: Averted. The holoemitter in Sickbay is able to make convincing decoys, but Book notes that neither they or their weapons will be real, so eight out of ten shots will be harmless.
- Internal Reveal: Averted with Mirror Spock. While Burnham reflects on her brother while aboard the I.S.S. Enterprise, she does not learn about his Mirror Universe counterpart, or Mirror Spock's role in reforming and ultimately dooming the Terran Empire, merely assuming that the mirror version of Spock was as ruthless as the rest of them.
- Lockdown: The phaser fight in Sickbay ends up triggering a lockdown that seals Burnham and L'ak inside, while Book and Moll make it to the doors before the forcefields come up.
- The Main Characters Do Everything: Justified. Rayner objects to Burnham taking on the potentially dangerous task of securing the next clue with only Book as backup when they know Moll and L'ak are there and may have secured it already, offering to go instead. Burnham explains that Book is their best chance for a peaceful resolution and she needs to go with him, and bringing anyone else would likely result in hostilities, which he begrudgingly accepts. It's hinted part of the reason he's trying to go in her place is that he'd rather not be left in charge of her crew, as he still hasn't completely adjusted to his new role.
- Mexican Standoff: Book briefly manages to talk Moll into a standoff, both parties holding each other at gunpoint. The fighting soon resumes when Moll loses her temper, resulting in Sickbay locking down with Burnham and L'ak trapped inside.
- Mortal Wound Reveal: As L'ak and Burnham fight in Sickbay, he grabs a shard of metal and threatens to stab her if she doesn't move. He charges and she flips him over, managing to steal the next clue. L'ak then gets up, revealing that he ended up stabbing himself in the process. Fortunately, even 23rd century Terran medicine is sufficient to keep L'ak alive.
- Number Two: Rayner steps into this role fully, leading the Discovery bridge crew in collaboratively solving the problem of wedging open the wormhole aperture. He has also embraced his place on the ship: when asked how it felt to sit in the captain's chair, he replies, "That's for the captain." And, indeed, throughout the entire crisis, he never sits down in it, instead supervising from over the chair's shoulder. Whether this is due to an acknowledgement of his role or an unwillingness to get too comfortable in the center seat, it's relatively unprecedented in the history of the franchise.
- Pocket Dimension: The seemingly empty area of space actually has a wormhole that leads to a pocket of interdimensional space.
- Ragnarök Proofing: The I.S.S. Enterprise survived 800 years in the Pocket Dimension and still retains power for life support, internal sensors, forcefields, and so forth. It's only when Moll deliberately engineers a power surge that everything starts to go to hell, and the ship still has a functioning tractor beam even after that. It doesn't take much effort for the Discovery crew to render her spaceworthy for a trip to the fleet museum, but they do have the benefit of an army of repair drones with 32nd century programmable matter.
- The Reveal:
- L'ak is Breen, in fact the first Breen seen in the franchise as previously they have remained fully suited and with indecipherable speech patterns. It's shown that Breen have two forms, one a translucent / luminous appearance that's maintained inside their suits and another more solid looking form when outside the suit. They consider the translucent form superior, and the other form sinking to the level of inferior life forms.
- L'ak and Moll are not seeking just money but attempting to buy their way out of a Breen Erigah, a type of bounty reserved for high crimes. L'ak is part of a royal family and engaged in a romantic relationship with Moll, when their romance was discovered he attacked his uncle and security guards before escaping. Moll also wants to escape to a safe haven in the Gamma Quadrant, which her father promised to take her to before just disappearing from her life without warning.
- The I.S.S. Enterprise had managed to escape from the Mirror Universe after Spock's attempts at reform led to the Terran Empire's conquest by the Klingon-Cardassian Alliance. She became stuck in a pocket dimension, and remained stuck there until Burnham and Book free her. Dr. Cho, one of her crew, eventually became a part of the research group that was studying Progenitor technology and was responsible for hiding the clue on her former ship.
- Save the Villain: When L'ak accidentally stabs himself during a struggle, Burnham passes Moll some medical supplies so she can treat him, then leaves the pair alone while she and Book deal with the more immediate problem of the ship about to crash into the wormhole opening, which it can't fit through.
- Series Continuity Error:
- Possibly. Burnham describes an Erigah as a Breen "blood bounty", and L'ak bleeds after getting stabbed — despite the fact that Breen were previously stated not to have blood (unless that was a misconception, along with the belief that their homeworld was a frozen wasteland). Alternatively, L'ak's conscious use of his solid form may be the reason for this, as Ruhn stated that L'ak was weaker and more vulnerable than the other Breen by using that state.
- Played straight with the I.S.S. Enterprise is presented in a mid-2250s configuration like she was on Discovery and Strange New Worlds, despite being in the more familiar TOS look in her last appearance in "Mirror, Mirror"—and it's doubtful that anyone had time to refit her back to that old look while they were trying to book it out of the Mirror Universe. That being said, it was far cheaper to reuse the existing SNW sets and models rather than recreate the TOS sets from scratch.
- Shoot Your Mate: L'ak's uncle hands him a blaster and demands he kill Moll to prove his loyalty. L'ak instead shoots him and his entourage, though non-lethally.
- Switch to English: The Breen speak in their typical Black Speech, which includes L'ak when he's first introduced in the flashback, but L'ak soon switches to English and gets his uncle to do the same later.
- Villain: Exit, Stage Left: After the Enterprise is brought through the wormhole, Moll and L'ak escape using a warp-capable probe left behind when the ship was abandoned.
- Wham Episode: Fan speculation on the true nature of the Breen and what they look like has been going on since DS9. In this episode, we discover the truth: L'ak is a Breen. The Breen themselves have two appearances they can shift between: one is a corporeal biological form, and the other is a translucent green form that is liquid or maybe non-corporeal as it is treated as a superior advancement from their fleshy form.
- Wham Shot: In the Pocket Dimension, Burnham and Book find a ship in the center: the I.S.S. Enterprise.
- Whatever Happened to the Mouse?: The ultimate fate of the I.S.S. Enterprise following the events of "Mirror, Mirror" is finally revealed. It crossed over to our universe prior to the downfall of the Terran Empire and was hidden in a pocket dimension in Tzenkethi space. It is Played Straight with most of her crew though, as nothing is ever said about what happened to Kirk's command crew after Spock reformed the Terran Empire.
- You Have Outlived Your Usefulness: Invoked when it's pointed out to Moll and L'ak that the Breen might just kill them anyway after taking the Progenitor technology, while serving a prison sentence with the Federation would be the safer choice. L'ak seems receptive to the idea, but only if he and Moll were allowed to serve their sentence together.
