Having returned from the Phantom Zone, Kara begins to have horrid visions about her experiences there as she tries to re-integrate back into her life. She also decides to spend time with her rescued father, Zor-El, and acclimate him to life on Earth. Alex, Nia, and Lena also wrestle with their own personal demons. Later, a crashed DEO satellite alerts Zor-El to a crisis happening in a garbage island off the coast of National City. He, Brainy, and Lena decide to use Kelex, the robot helper inside the Fortress of Solitude, to clean up the garbage, since the wreckage included Kryptonite, so Kara couldn't do it. However, something goes wrong and Kelex ends up turning into a giant garbage monster that needs to be stopped.
Tropes:
- Attack of the 50-Foot Whatever: The garbage monster that Oscar turns into after accidentally bonding with a Teallian is quite big.
- Ascended Meme: Andrea officially christens Supergirl and her buddies as the "Superfriends."
- Beam-O-War: Oscar the garbage monster and Nia engage in one.
- The Bus Came Back: Andrea and William return in this episode.
- But Now I Must Go: Zor-El leaves at the end of the episode to reunite with his wife on Argo City.
- Chekhov's Gun: J'onn tells Kara that the DEO satellite had Kryptonite on it, preventing her from solving things herself easily. When the garbage monster attacks later in the episode, it uses said Kryptonite to shred Kara's armor and make her weak, requiring the others to save her.
- Colony Drop: A defunct DEO satellite drops out of orbit. Kara pitches it back out into space, but some of its flaming debris ignites a DEO garbage pile in the ocean, which then becomes the problem of the episode.
- Continuity Nod:
- The various fears the characters went through last episode get brought up again.
- Kara tells her dad that Clark showed her around the Fortress of Solitude.
- Kara brings up Iris's article about the recovery of Central City from the Mirrorverse invasion from The Flash (2014).
- When Andrea lists the top national news media in the country, The Daily Planet, Gotham Gazette, Star City Sentinel, and Central City Citizen are all on the list.
- Contrived Coincidence: The DEO satellite had Kryptonite hidden inside, meaning Kara can't simply go around and clean up the garbage island herself.
- Da Editor: Andrea becomes one of these for Kara, ordering her to cover the Phantom attacks from last episode as a makeup to missing so much time with nothing to to show. When Kara comes back with a pitch for a potentially dangerous environmental waste story, Andrea denies her the chance to cover it and hands it to William instead. Finally, after William finishes up that story, she decides to assign him to do a deep dive on the Superfriends.
- Dangerous Phlebotinum Interaction: While cleaning up the DEO garbage pile, Oscar fuses with a lifeform called Teallian in the trash that causes it to attract garbage like a magnet, resulting in it turning into a giant, rampaging monster. Once freed from the garbage, the robot goes back to normal.
- Double-Meaning Title: "Welcome Back, Kara" applies in-universe to the other characters happy to have their friend back. It also has a meta reasoning since it's the first episode back from an extended hiatus.
- Fish out of Water: Zor-El has to adapt to Earthly customs and his newfound powers due to Earth's yellow sun.
- Freudian Slip: Zor-El says Oscar can "still save Krypton — Earth. He can still save Earth."
- Going for the Big Scoop: What the others established as Kara's cover story to explain her absence while trapped in the Phantom Zone. She was supposed to be going underground to get close to a Middle Eastern resistance fighter, though Kara deflates the cover almost immediately afterwards.
- Green Aesop: This episode is partially dedicated to human pollution of the planet, particularly plastic pollution in the oceans.
- Heroic BSoD: Kara, Alex, Lena, and Nia all experience various levels of PTSD after the events of last episode, though it's most obvious in Kara.
- Hurl It into the Sun: How Kara disposes of the garbage monster's body.
- Internal Reveal: Kara learns about the Phantom attack on the city during her absence.
- Loophole Abuse: Due to having a third party dispose of the waste, Lex found a loophole in government regulations and thus technically didn't do anything illegal.
- Mean Boss: Andrea threatens to fire the entire staff if their national news ranking doesn't improve in a year.
- My Greatest Failure: Zor-El is still blaming himself for Krypton's destruction and panics when he thinks Earth is going down the same path.
- Mythology Gag: When Andrea gives William his new assignment, the camera pans over to previous CatCo articles written about Supergirl. One of them uses the cover art from the first DVD/Blu-Ray set of the series.
- The energy being Oscar bonds with is a Teallian, the same species as Legion of Super-Heroes member Quislet.
- Obvious Stunt Double: Supergirl dons her anti-Kryptonite suit during the final battle, so of course this trope is in effect.
- Older Than They Look: When Kara takes Zor-El to Catco, she and Nia introduce him as Kara's "Uncle Archie," since due to his slowed Kryptonian aging, he looks more like an older brother rather than her father.
- Out of Focus: Nyxlgsptinz only appears in the recap in the beginning and Nia's nightmare at the end of the episode.
- Put on a Bus:
- Lena leaves at the end of the episode to find out what happened to her real mother.
- Zor-El goes off to Argo City to be with Alura again, and Brainy takes him there.
- Running Gag: Once again, Alex contributes little to no actual assistance in the field when battling the giant trash monster. She tries to save Kara from being killed, but her gun is ineffective and it's up to Dreamer to actually save them both.
- Ship Sinking: William breaks off his pursuit of Kara, telling her that he's seeing a pediatrician named Mary.
- Shout-Out:
- When Kara wakes up from her coma, she references The Phantom of the Opera, The Phantom Menace, and Scooby-Doo.
- Zor-El renames Kelex "Oscar" after upgrading him, in reference to Oscar the Grouch.
- The Stinger: Nia wakes up to a nightmare about Nyxlgsptinz.
- Trespassing Hero: Andrea once again dons her Acrata outfit in order to sneak into Lex's office and find incriminating evidence that the DEO satellite really did release hazardous waste into the ocean.
- Written-In Absence: Kelly is said to be busy doing social work and doesn't appear.
- You Fight Like a Cow: Brainy riles the garbage monster up in order to get it to absorb him and destroy it from the inside. It works. Also a Stealth Pun: He's trash-talking the trash monster.
