With the war going badly, Starfleet and the Federation authorize a daring and risky plan to put the Klingons on the defensive. The crew come to grips with what happened while they were in the Mirror Universe.
Tropes in this episode include:
- Accuser of the Brethren: While a fair portion of the crew starts the process of rapprochement with Tyler, Stamets meets him only with cold fury and tells him he hopes that Tyler feels sickened and anguished over killing Culber.
- All Your Base Are Belong to Us: Cornwell orders Discovery to set course to Starbase 1, but the Klingons have seized it by the time they arrive. On a broader scale, 20% of former Federation territory is now occupied by the Klingons and they're poised to move on Earth.
- "Awkward Silence" Entrance: Tyler gets this in the mess hall, now that the truth about him is known. Tilly pointedly gets up and goes to sit with him, breaking the tension.
- Bad Guys Do the Dirty Work: Implicitly, this is why Cornwell has agreed to put Mirror Georgiou in command of Discovery. She possesses the ruthlessness necessary to definitively end the war with the Klingons, while the Federation does not.
- Blood Knight: The Klingons, naturally. L'Rell pointedly tells Cornwell that the Klingons have tasted blood in their fight with the Federation and won't ever stop unless they're conquered in turn.
- Boarding Party: Admiral Cornwell, Sarek and a security detail beam over to secure Discovery, because to them the ship was destroyed nine months ago.
- Call-Back: It's mentioned that no Starfleet vessel has visited Qo'noS since Jonathan Archer did with the Enterprise NX-01 a century earlier.
- Cannot Spit It Out: Sarek still can't quite openly call Burnham his daughter, but he's getting closer, referring to his wife Amanda Grayson as her mother.
- Cliffhanger Copout: The previous episode ended with Saru looking at a star map and declaring that the Klingons have won the war. In the beginning of this episode we find out that while the Federation has suffered some heavy losses, the war is far from over.
- Continuity Nod:
- Cornwell refers to the events of the very first episode of Star Trek: Enterprise, "Broken Bow".
- Retroactive: Cornwell and her officers discuss that Starfleet intends to keep knowledge of the Mirror Universe secret, explaining why Kirk and his crew in Star Trek: The Original Series have no knowledge of it in the episode "Mirror, Mirror".
- Sarek mentions that Burnham's relationship with Tyler, effectively a human and a Klingon in love, gives hope that the Federation and the Klingons might similarly find peace with each other. Their cold war would end in Star Trek VI: The Undiscovered Country and the two powers would be allies by Star Trek: The Next Generation.
- Dead Person Impersonation: Mirror Georgiou works out a deal with Sarek and Cornwell to take the place of her deceased Prime counterpart—who they claim was wrongly presumed dead aboard T'Kuvma's ship, and was actually captured and held prisoner by the Klingons until a recent "rescue"—in exchange for teaching them how to win the war against the Klingons. Somewhat subverted, in that Starfleet, Burnam, and everyone else on Discovery responsible for rescuing Mirror Georgiou know that she's the Terran emperor and not their Georgiou. But a lot of people were left out of the loop.
- Decapitation Strike: On Mirror Georgiou's advice, Starfleet plans to do this to the Klingon Empire, assaulting Qo'noS and devastating its defenses and surface installations to force the Klingons to withdraw back into their territory. However, while this is the official line, Mirror Georgiou's deal with Sarek suggests there's a much darker plan in store.
- Didn't See That Coming: Sarek remarks that nobody could have predicted that the reason for Lorca's bizarre, Ambiguously Evil behaviour was that he was from a parallel universe.
- Evil Power Vacuum:
- With both T'Kuvma and General Kol dead, the Klingon Empire has once again divided into its component houses, all vying for power and influence. Unfortunately, they're doing this by attacking the Federation to see who can gain the most glory, leaving the Federation fighting a losing war against 24 different enemies that have no logical pattern or methodology in their attacks.
- It's also mentioned that with Georgiou missing, most of her senior officers dead, and the palace-ship Charon destroyed, the Terran empire back in the Mirror Universe is likely in its own bit of chaos.
- Five-Second Foreshadowing: Saru's threat ganglia emerge shortly before Discovery drops out of warp at Starbase 1, so you know something bad has happened.
- Foregone Conclusion: Even though Mirror Gergiou suggests ending the Klingon threat by doing something horrible to Qo'noS, we already know that they likely won't be doing anything that bad to the planet, since it's still populated in the series and films to come.
- Heroic BSoD: When Cornwell sees that Starbase 1 has been seized and its 80,000+ crew slaughtered to a man, she's left unable to even act. Saru has to order their escape before the ship is fired on.
- Killed Offscreen: The ISS Discovery was apparently destroyed by the Klingons not long after the USS Discovery swapped places with it. By extension, this also means that Mirror Tilly (probably along with a few other Discovery crew counterparts) died, leaving the mirror versions of Detmer and Saru as the only known surviving counterparts from the show's main and recurring characters.
- Liar Revealed: When Mirror Georgiou recognizes Saru as Kelpian and admits to having eaten his kind, Burnham is forced to admit to lying about that to spare his feelings. He lets it slide, given the circumstances.
- Love Cannot Overcome: Though both Tyler and Burnham love each other, Burnham can't be with him after seeing him try to kill her as Voq.
- Mind Probe: Cornwell has Sarek mind-meld with Saru to determine if he's really who he claims to be, since from their perspective Discovery was lost with all hands nine months ago.
- Mythology Gag: Terraforming a dead moon in the blink of an eye? It's the Genesis Device, twenty-five years early.
- No One Could Survive That!: Invoked by Cornwell with regard to the prime universe's Lorca, as she sadly realises that the chances of him surviving for very long in the Mirror Universe, especially when his counterpart was the most wanted man in the Terran Empire, are slim to say the least.
- Not So Different: Mirror Georgiou has this opinion of both versions of Burnham, considering them women of great potential who made a single mistake that ruined everything.
- Not So Similar: On the other hand, Sarek points out that his Burnham helped defeat the man who seduced Georgiou's Burnham.
- Override Command: Cornwell issues a verbal command to override Discovery's command codes and place herself in charge when boarding the ship, as she isn't convinced they're who they say they are.
- "The Reason You Suck" Speech:Georgiou: Why did you bring me here? Truly.
Burnham: I wanted to show you a place of morality, of hope.
Georgiou: Let me tell you, daughter who is not my daughter. In truth, you wished to save me because you couldn't save [your Georgiou]. This regret that you have for what you did, it weakens you.
Burnham: I feel it every day of my life.
- Redemption Quest: Burnham says that despite everything she has done so far to atone for her actions at the Battle of the Binary Stars, she still has a long way to go to redeem herself.
- Scenery Porn: The terraformed moon. Well done, Mr. Stamets.
- Sci-Fi Writers Have No Sense of Scale: The Discovery needs to reach Starbase One and we're told this will be difficult because the sector is crawling with Klingon ships. There are two problems with this scenario. The first is that Discovery is stated to be only one light-year away from Starbase One which, in interstellar terms, would mean they're practically there already. Secondly, Starbase One is said to be 100 AUs from Earth, which would put it on the outer edge of the solar system (specifically, twice as far as Pluto's aphelion). But we very quickly learn that Starbase One has been overrun by Klingons, which would put the solar system — and Earth — under direct threat of attack. Yet Starfleet Command acts as if this was just another unfortunate defeat, not an immediate existential threat.
- Suicide Attack: During Discovery's absence, the Klingons devastated the Federation's fleet and shipbuilding capacity by having Klingon ships under cloak tail damaged Starfleet vessels back to their shipyards and then self-destruct.
- Terraforming: In need of a new spore supply and lacking time to cultivate one in a lab, Stamets instead takes his stock specimens and loads them into probes so they can be fired at a dead moon. EM pulse charges are then used to energize the samples and cause them to rapidly multiply across the entire surface.
- That Satisfying Crunch: Cornwell is so enraged by Mirror Lorca's deception that she vaporizes the bowl of fortune cookies he kept on his desk while Saru, Stamets and Burnham are sitting in the line of fire about three feet away.
- Tyrant Takes the Helm: Cornwell gives command of the Discovery to the "recovered" Captain Georgiou, with only her, Saru, Burnham, and the senior crew knowing the truth.
- Understatement: The Vulcans are still rather good at it.Sarek: That Lorca was an impostor from an alternate universe was not the most obvious conclusion.
- Virtue Is Weakness: Emperor Georgiou says that Burnham's regret over not being able to save Captain Georgiou "weakens" her.
- Wham Shot: Discovery drops out of warp at Starbase 1—and sees the crest of House D'Ghor crudely painted on the outer hull of the base.
- Worthy Opponent: Cornwell and L'Rell certainly don't like one another, but they clearly hold each other in a great deal of respect. L'Rell is even willing to admit that her messiah T'Kuvma was wrong to claim humans are entirely lacking in courage.
- You Didn't See That: Saru makes it clear that the transporter chief did not just beam aboard Emperor Georgiou and that any mention of what they did not do will carry the same punishment as treason.