James and the DEO try to track down a rogue telekinetic alien. Meanwhile, Rhea furthers her plans with Lena's help.
Tropes in this episode include:
- Alien Invasion: It turns out the portal Lena and Rhea are working on is a means for Rhea to bring her forces to Earth.
- Always Second Best: Rhea cleverly makes Lena think that she can surpass Lex, something she always wanted to prove.
- Blatant Lies: Rhea claims Lar Gand killed himself out of despair when Mon-El left.
- Brainwashed and Crazy: A side effect of the scaled-up portal technology causes the telekinetic aliens to lose control and wreck everything in their path.
- Broken Pedestal: After openly stating that Reha is like the mother she's never had, Lena is rocked to realize Rhea was using her all this time to facilitate an alien invasion of Earth.
- Call-Back:
- James still has his broken camera.
- Mon-El brings a gun to threaten Rhea, recalling how Cadmus shot him.
- Crazy-Prepared: Since their last encounter, Rhea has obtained a Lotus-Eater Machine device from the White Martians to neutralize J'onn.
- Creepy Child: Marcus initially seems so, especially when his powers go out of control.
- Continuity Nod: Winn whistles "Superfriend" from the "Duet" crossover with The Flash (2014).
- Cutting the Knot: James shows Marcus a mechanical tilt maze with the objective of getting a ball from a start to endpoint without dropping it into any holes, and he invites Marcus to try it out. Marcus just uses his telekinetic abilities to move the ball around the obstacles.
- A Day in the Limelight: This is the most James-centric episode of the season and comes after a string of episodes where he barely even appeared.
- Double-Meaning Title: The titular lost children refers to:
- The alien inhabitants of Earth, particularly the refugee telepathic Phorians.
- Very literally Marcus (lost from his mom) and also his mother (lost to the authorities and Superfriends).
- The Daxamites who plan to make the planet their new home.
- Mon-El, supposedly lost to Rhea.
- Lena, especially and explicitly in this episode.
- James, aiming to find his real heroic purpose.
- The alien inhabitants of Earth, particularly the refugee telepathic Phorians.
- Evil Is Petty: Rhea lets Supergirl know that everything she will do to her planet is because of their feud.
- Evil Mentor: As Lena lampshades, Rhea plays a mentor role to her, even acting as a Parental Substitute at times. Rhea insists she does care, but she's still using Lena for her own ends and doesn't even bat an eye when she gets knocked unconscious.
- The Ghost: We get another reference to "Clark's friend/frenemy," another vigilante who's apparently a bit of a Terror Hero.
- "I Know You're in There Somewhere" Fight: James manages to talk down Marcus while the portal device is affecting him and the others. Since they share a telepathic connection, stopping one stops all of them.
- Lotus-Eater Machine: Rhea neutralizes J'onn with a White Martian device designed to trap him in an endless nightmare.
- Multi-Character Title: The titular lost children refers to the alien inhabitants of Earth and the Daxamites who plan to make the planet their new home.
- Mythology Gag: When James is venting out his frustrations of being a Hero with Bad Publicity to Winn, he compares himself to another one of "Clark's friends," followed by Winn making a "pointy-eared" gesture to his head.
- Never My Fault: Rhea absolutely refuses to consider that Mon-El did not want to go with her and was only pressured into it, and continues to blame Supergirl for alienating him from her. She also tells Supergirl that children will scream and die because Kara was selfish.
- Oh, Crap!: Kara has one of these looks when she realizes what the portal's for.
- Parental Substitute: Tying in with the lost children, James becomes one for Marcus, in a way, and Rhea for Lena. Also J'onn for Kara, as always.
- Person of Mass Destruction: The Phorians have some devastating telekinetic abilities, though they're a peaceful race ordinarily. Unfortunately, they also happen to be vulnerable to drastic shifts in mental state as a side effect of the portal Lena and Rhea are testing.
- The Place: The titular city is obviously National City.
- The Quiet One: Marcus doesn't speak a lot.
- The Reveal: Rhea helped Lena build the portal to bring the remnants of her scattered people to Earth and turn it into New Daxam.
- Thousand-Yard Stare: Marcus's default facial expression.
- Unwitting Pawn: Lena finds out too late that Rhea just played her.
- "Well Done, Daughter!" Girl: Rhea recognizes that Lena really wants a mother-figure and takes full advantage of that to manipulate her. On the one hand Rhea does genuinely seem to see Lena as the daughter she never had—but as her relationship with Mon-El shows, that's not really a good thing.
- What the Hell, Hero?: James scolds J'onn for the way they are locking Marcus up.
- Writing Around Trademarks: James and Winn talk about a certain friend of Superman who is a fellow hero but inspires fear. That's obviously Batman, which is made more obvious by Winn making a gesture of his iconic cowl, but copyright issues prevented them (and the show) from actually namedropping him.
- You Are Better Than You Think You Are: J'onn gives one to James, who later gives one to Marcus.
- You Wouldn't Shoot Me: Rhea correctly predicts that Mon-El won't shoot her because he still cares for her.
