Eliza comes to town to celebrate Thanksgiving with her daughters. Alex decides it's time to come out to Eliza, Eliza suggests to Kara that Mon-El likes her, James and Winn consider coming clean to Kara about the Guardian, and a breach in the space-time continuum appears above the table. These all turn out to be the least of their problems when they find out that CADMUS plans to unleash an alien-killing virus across National City.
Tropes in this episode include:
- After Action Patch Up: After Maggie is shot by Henshaw, Alex stitches up her wounds back at the DEO headquarters. Alex also uses this time to thank Maggie for helping her realize her sexuality since she feels like she finally understands herself now because of that realization.
- Alien Among Us: Poor Mon-El tries very hard to get into the holiday spirit, wishing everyone a "Jolly Thanksgiving" and handing Kara a sack full of fluff.Kara: What's this?
Mon-El: Oh, you asked for stuffing, so I ripped open my mattress and pulled some out.
- Antagonist Title: Medusa is the bioweapon that Lillian Luthor plans to use to kill all aliens on Earth.
- Big Damn Heroes:
- Subverted during Henshaw's fight with Supergirl. The breach appears, making the viewer think Barry is about to emerge and lend a hand (right after Henshaw Tempts Fate by saying, "No one is coming to save you"), only for it to close suddenly. Still, it distracts him long enough for Supergirl to regain her footing. Barry instead shows up at the end, after the main conflict with Cadmus is resolved.
- Mon-El gets a nod, since as others are running out of the bar, he is running back in to try to help. Granted, he has partial immunity to the virus due to his Daxamite physiology (partially Kryptonian), though he had no way of knowing that at the time.
- The Big Damn Kiss:
- After Maggie almost died, she admits to Alex that she has feelings for her, and they kiss.
- Mon-El kisses Kara when he is dying from the virus, but claims to not remember this after he is cured.
- Bookends: To the final chapter of the crossover, as both episodes deal with a bioweapon that targets a specific group—aliens in the case of Medusa and metahumans in the case of the Dominators' weapon.
- Broken Pedestal: Kara grew up thinking her father worked to save lives, not harm people. His avatar clarifies, "Kryptonian lives."
- Bury Your Gays: Averted. Maggie gets shot, but recovers quickly.
- The Bus Came Back: Lena returns, having not been seen since the 5th episode.
- Central Theme: Our parents may be horrible people or do horrible things with good intentions, but we as their offspring can do better and be heroes.
- Chekhov's Gun: Averted with relation to the crossover. It was widely speculated that the bioweapon used to kill aliens in this episode would be employed by Supergirl to defeat the Dominators, but it ultimately is not (it's not even mentioned as an option).
- Complexity Addiction: A Discussed Trope with regards to Lillian Luthor.Alex: Why did Lillian Luthor take your blood? I mean, why not just kill you?
Kara: I... think she has something a little more nefarious planned.
Winn: Ah yes, those Luthors do love an epic criminal scheme.
- Coming-Out Story: The third episode in a row dealing with Alex coming out to someone. Alex through the first half of the episode is trying to build up the nerve to tell her mother that she's gay, only for it to be revealed that Eliza pretty much figured it out by the way her daughter was acting and their conversations about Maggie, and she reassures her daughter that she is okay with it.
- Continuity Nod:
- Barry reminds Kara of their deal that she would come to help him when he needed it next.
- The aliens searching for Mon-El have tracked him down as far as the Well of Stars before losing the trail. Kara had previously speculated that Mon-El's stay in the Well, where Time Stands Still, was the reason why he hadn't aged since Krypton's destruction.
- Winn says that a wormhole opening above the table was only slightly less intense than what happened last Thanksgiving.
- Cisco is also seemingly still bitter about Barry indirectly causing the death of his brother (which he found out in the episode "Killer Frost" that aired before this one), as he is quick to correct Barry when he calls Cisco his ''friend''.Cisco: "Friend" is a loose term; we work together.
- Curb-Stomp Battle:
- Henshaw disposes of Mon-El quite easily after poisoning the alien bar.
- He also makes good work of Kara later, though she gets a few good hits in first.
- At the climax of the episode, Henshaw also hands one to J'onn — before the latter turns into a White Martian and gives it back to him (only being stopped by a sucker attack).
- Darkest Hour: Cadmus has taken a virus that will kill all aliens and plans to disperse it, Mon-El is already dying from it, and J'onn is slowly turning into a White Martian with no seeming way to undo it; meanwhile, the real Hank Henshaw constantly proves to be an enormously powerful cyborg (possibly more than other cyborgs seen thus far like Reactron and Metallo) and constantly bests the heroes in battle, while Lillian always hides her tracks.
- Death Seeker: J'onn would rather die on a suicide mission than live as a White Martian.
- Determinator: The aliens pursuing Mon-El have no intention of giving up the search... ever.Navigator: My Lords. We have reached the Well of Stars.
Hooded Alien: And?
Navigator: There is no sign of the Kryptonian pod.
Hooded Alien: Find it.
Navigator: The ion trail has dissipated. I would not know where to look.
Hooded Alien: Find it. I do not care how much space we must cross, how many lives we lose, or how many suns we burn. Mon-El of Daxam will be found.
- Fake Defector: After being told that Lillian is the head of Cadmus, Lena calls her up and joins the cause, offering the isotope necessary to stabilize the Medusa virus. Turns out, she gave Lillian the wrong isotope, rendering the virus inert, and called the police to arrest her.
- Fantastic Racism:
- Cadmus's plan is to use Medusa to wipe out all of the aliens. Henshaw throws insults at Mon-El, Kara, and J'onn when he is fighting them.
- Kara's father created Medusa as an anti-invasion deterrent, designed to kill any alien life that attacked Krypton while leaving the native population unharmed. His hologram doesn't seem particularly bothered by it, justifying it as saving Kryptonian lives. While Kara is horrified, the crossover establishes that the Dominators previously attacked Krypton, so his willingness to develop such a bioweapon is understandable in that context.
- Because of his hatred for White Martians, J'onn would rather die than become one. It also seems like he hasn't told anyone he has M'gann locked up, or it is known and they just accept it because of her being a White Martian.
- Final Solution: What Cadmus intends to do with the Medusa virus, killing all extraterrestrial life. You'd think they'd live over a wider area than a single city, but there wouldn't be anything to stop them from repeating the process elsewhere.
- Funny Background Event: After Kara takes Alex's beer away before the Thanksgiving dinner and puts it in the freezer, she converses with Eliza. While Kara and Eliza are talking, Alex walks to the freezer, takes out a bottle of whiskey that was sitting next to the beer, and sneakily runs it back to the table.
- Heel–Face Revolving Door: Played with. Lena performs the "Face-Heel" variant as she appears to turn bad in order to trick her mother. However, given that she is a Luthor, her true motivations are still not completely clear.
- I Am a Monster: J'onn admits this to Hank before transforming into a green version of the physiology of a White Martian to beat him up bare-handed. Doubles as a Pre-Asskicking One-Liner.
- The Immune: Zig-zagged; Medusa doesn't instantly kill Mon-El like it does other aliens, since Daxamite and Kryptonian biology are pretty close... but not close enough that it doesn't take a toll on him. Humans are shown to be completely immune when the humans at the alien bar are able to flee without even becoming ill, presumably thanks to Cadmus tweaking it for their purposes.
- If You're So Evil, Eat This Kitten!: To prove her loyalty to Cadmus, Lillian tells Lena that she needs to launch the Medusa-carrying missile to wipe out aliens on Earth.
- I Knew It!: Kara quotes the trope name when she comes home to find Barry and Cisco, having recognized the similarity between these dimensional breaches and the one Barry opened when he left last time.
- Internal Reveal: Defied by Alex, who doesn't want her own Coming-Out Story at Thanksgiving to be ruined by James and Winn telling Kara about Guardian.
- Kick the Dog:
- In addition to being a genocidal maniac, it's clear Lillian was never a great mother to Lena either, and even admits that she preferred Lex.
- In a meta example for Flash fans, Cisco's line about him and Barry not being friends brings up an unpopular plot development from that show.
- Kryptonite Factor: The Medusa Virus is designed to be a literally universal weakness for pretty much any non-Kryptonian species; the only reason it doesn't affect humans is because of Cadmus's modifications.
- Mirror Match: J'onn, impersonating Hank, faces the real Hank at the climax of the episode. This is an unusual example of this trope, given that the impostor is actually the good guy while the real one is the bad guy.
- The Multiverse: While its inhabitants still think they are on the "one Earth," this world has been officially numbered Earth-38, in the Arrowverse multiverse. In the final episode of the "Invasion!" crossover, Cisco gives Kara a device that can not only open a portal to Earth-1, but also will allow them to communicate back and forth.
- Mythology Gag: J'onn's green "White Martian" form resembles his DC Animated Universe incarnation's true form.
- Never Trust a Trailer: The ads for the Arrowverse's big crossover event all claimed this episode was part of it, despite Marc Guggenheim being completely open before the season even started that it wouldn't be connected beyond Kara heading to Earth-1 at the end. In the final episode, the only direct reference to the crossover are the momentary appearances of Barry and Cisco's breach, a less-than-a-minute-long sequence at the end where Barry and Cisco finally appear, and, thematically, this episode has similarities to the finale of the crossover as both involve human distrust of aliens and a bioweapon that targets a specific group for extermination.
- Nice Job Fixing It, Villain: Lillian's smug assumption that Lena is as bad as she is allows Lena to sabotage the virus, get Lillian arrested, prove that Kara's faith in her being a good guy is legitimate, and provide a means for Eliza to create an antiviral cure and a cure for J'onn.
- "Not So Different" Remark: After finding out about her father creating Medusa (combined with the revelations about her mother a season earlier), all for the supposed good of their race, Kara starts seeing a few too many similarities between her parents and Lex and Lillian Luthor.
- Oh, Crap!: Henshaw walks and talks like a real badass, but when J'onn morphs into a White Martian, he simply stares in pure terror.
- Parental Favoritism: Lillian always preferred Lex over Lena, with her husband seeing it the other way around.
- Pretender Diss: Kara tells Hank, "You may be a cyborg, but you're no Superman."
- Red Skies Crossover: "Part 1 of a four-night event" is how it was advertised; in practice, it's just a lead-in to the actual crossover spread across the other three shows. It doesn't even use the shared title. The episode is mostly focused on resolving the plot with Cadmus, and Barry only shows up with Cisco at the very end to recruit Kara. And coincidentally, there are red skies.
- Running Gag: Cisco and Barry try two or three times before they can get a breach to manifest and actually open to Kara's Earth.
- Sequel Hook: The episode features two:
- After the Medusa Virus is destroyed, an unknown group of aliens is shown to be searching deep space looking for Mon-El of Daxam.
- The episode ends with Barry and Cisco appearing in Kara's apartment asking for her help.
- Spanner in the Works: Lena Luthor, who successfully foils her mother's plans.
- Synthetic Plague: Medusa is a bioweapon designed to kill all non-Kryptonian life forms and Cadmus redesigned it to not harm humans.
- Wham Episode: Lillian is finally arrested (although Hank escapes) and Cadmus is seemingly defeated for now as a result. Also, some aliens want Mon-El, they'll look until they find him no matter how long it takes.