The episode opens with Guardian saving a woman from a pair of attackers — but when he turns to reassure her, she panics and runs, scared of him.
The next day, Kara is out to lunch with Lena, who tells her that she’s working on an exciting new project with a partner who is like a mentor to her…but can’t tell her anything more about that. She gets a call from the partner, though, and has to run.
In a park, Winn approaches James to talk about what happened last night. Comparing himself to Batman, he tells Winn that Guardian isn’t inspiring anyone; he’s making people scared. As they’re having the conversation, though, a young woman appears and starts to use psionic powers to throw people and objects around. Supergirl arrives to stop her, but the alien disappears.
Back at the DEO, she’s identified as a Forrian, a usually-peaceful race. The DEO declines Guardian’s help, sending James away frustrated.
At dinner, Rhea is trying to reassure Lena, who is frustrated by failures in the testing process on their transmitter bridge. She wonders what will happen if she can’t make it work, but Rhea says she has confidence in her. Rhea leaves, and walking nearby, Mon-El thinks he sees her but is distracted by traffic on the street.
In an alley, Guardian breaks up a drug deal involving an alien, who tells him that he’s only buying a little pot, but he can give James the location of the woman who attacked Simmons Square earlier. James heads to her apartment, where he sees drawings on a table and photos reflecting a normal home life with kids. A little boy comes home and is terrified to see Guardian there, but James takes his helmet off and tells him he’s only there to help.
At The DEO, Alex is trying to get information out of Marcus, the little boy. He's not saying anything, and in another room, James says he thinks this is the wrong approach. Eventually Alex figures out that the kid might trust James, and offers to send him offsite to talk with Marcus.
Mon-El heads to see Winn about some help with a satellite.
At L-Corp, Lena continues to be frustrated by failures, saying that she's frustrated because she thought this could help prove she's as smart as Lex. Rhea tells her to stop thinking like Lex and try to figure out a better work-around for herself.
At CatCo, James brings Marcus in and introduces him to Kara, but he still doesn't want to talk to anyone but James. James has Miss Tessmacher get them some milk shakes and curly fries, and brings Marcus into Cat's/his office. After looking around, he sits in front of a camera that's in pieces on a small table. James tells him that he bought the camera with his first paycheck from photojournalism; another one on the table is the camera his dad gave him, and when he starts sharing stories about his dad, Marcus opens up, saying that he lost his father, too.
Mon-El brings Kara Chinese food and tells her that he thought he saw his mother last night, but that he went and checked and his parents definitely left for home. He tells Kara that he hates that he still cares about his mother. Kara tries to console him.
At L-Corp, Lena is ready for the next test, and Rhea is there eager for it.
At CatCo, James teaches Marcus how to use his camera, but when Marcus goes to take a picture of James, he sees a news report about his mother. James asks where she might be, but before Marcus can say anything, Eve shows up with the food. While James talks to her, suddenly Marcus zones out and starts using his mental powers to wreak havoc the same way his mother had.
Supergirl speeds in and flies Marcus out of the building, holding him in the air while he has an episode. His powers seem to act up in proportion to the amount of energy being brought through the get at L-Corp, and it shuts down when Rhea turns down the controls. She brings the boy back to the DEO where they can put him in a cell to dampen his powers and stop him from having another episode. James tries to run, frustrated, but J'Onn talks him into sticking around with a pep talk.
Winn figures out that a spike in a particular sort of particle in the atmosphere has been driving the Forrians' powers crazy; Kara realizes that the particles were mentioned during her conversation with Lena, and calls Lena to let her know there's a problem -- but Rhea answers Lena''s phone.
Rhea accuses Supergirl of creating conflict when she came in peace, and says that anything she does will be Supergirl's fault. Mon-El tries to talk her into stopping, but she doesn't want to hear about it. She hangs up and deletes the call from Lena's history.
In Marcus's cell at the DEO, Marcus says he's worried that Earth will kick him out like the other worlds where he and his mother have tried to settle down. James tells him that he had a pretty bad childhood after he lost his dad, too, and that he had coped by blocking people out. He tells Marcus that it was Clark (not Superman) who helped him to open up and trust people again.
Marcus tells James that he can see his mom, but that he doesn't know where she is. He says that he can take James to her, and James promises to protect them. J'Onn says he can't let them take Marcus offsite, but James convinces him after Winn says there's a mobile version fo the telekinetic dampener that should keep either of the two from having an episode.
Marcus leads them to his mom, who is paranoid when she sees people there. Marcus tells her not to worry, but there are more than just two aliens there.
At L-Corp, Lena is worried about the idea of Rhea leaving, but Rhea tells her that whatever happens next, she should know that she's a marvel, and any mother should be proud to have her. She turns the gateway on as high as it goes, and Lena starts to panic. At the DEO, Supergirl and Mon-El see the signal and know where to go confront her -- after Mon-El gets something. And in the alien hideout, the psionic dampener isn't powerful enough to protect James and Winn from a room full of "activated" powers.
At L-Corp, nothing Supergirl and Mon-El do can close the gateway, and Rhea is bringing something to Earth. While Supergirl struggles to break the gate, Rhea uses White Martian technology to trap J'Onn in a nightmare. Elsewhere, James tries desperately to get through to Marcus, so that Marcus can calm the other aliens. Just as he does that, Mon-El holds a gun on her, reminding her that they aren't bulletproof here. She talks him into putting it down, telling him that Lar Gand took his own life after Mon-El left. As he lowers the gun, she opens a gate for thousands of Daxamite refugees in hundreds of ships. Across the city, everyone watches as ships descend on Natioanl City and Rhea says, "Welcome home to New Daxam."