Armek, a White Martian, descends on National City intent on taking M'gann back to Mars to face her punishment as a traitor. Hank and Supergirl determine the best way to keep M'gann safe is to bring her to the D.E.O.. However, when it is revealed that Armek shape-shifted into M'gann and is now loose in the building, the team realize he could be disguised as any one of them.
On her 13th Earth Birthday, Kara arrives at the Al's Dive Bar, and Mon-El gives her a club soda with rocks. Mon-El is still trying to get a relationship with Kara, but Kara is not convinced they would be compatible (likely due to them being a Daxamite and a Kryptonian, respectively). Alex arrives and Kara tries to get her to celebrate her birthday, but Alex says she has plans with Maggie - to go to a Barenaked Ladies concert. Kara tells them to go. M'gann sees that Kara is feeling down. When M'gann is taking trash outside, J'onn shows up. Though M'gann claims she can take care of herself, J'onn insists on looking after her. When M'gann says there are no White Martians here, they are suddenly attacked by one. Kara comes to their aid and it flees.
J'onn insists keeping M'gann safe at the D.E.O., but she refuses. Back at the bar, M'gann gets a visit from her former mate, Armek. He gives her two hours to surrender, or else he starts hurting her friends. M'gann stands her ground and Armek leaves. Alex is feeling guilty, having turned Kara down, which Maggie notices. Maggie urges Alex to talk with Kara.
M'gann tries to leave in an attempt to keep her friends safe, but J'onn convinces her to let them protect her, and takes her to D.E.O. At the D.E.O., Kara is surprised that Winn is still at work, thinking he was helping Guardian with something. Kara shows her concern for Winn, because she doesn't want to see him hurt, but Winn claims that it is his calling. Alex talks with Kara, but Kara tells her to be with her girlfriend. J'onn and M'gann arrive, but soon after a second M'gann arrives. Everyone soon realizes that the "M'gann" that came with J'onn is actually Armek in disguise. He attacks J'onn and Kara, and the fight causes the power to go out. Winn manages to restore auxiliary power, but Armek is gone. J'onn locks down the building to keep Armek trapped. However, since the White Martians can adopt anyone's appearance, all people inside now face a problem: which one of them is actually Armek? J'onn is unable to sense it due to psychic interference.
As the D.E.O. agents start accusing each other of being a White Martian, J'onn breaks it up and reveals that there is a way to reveal a white Martian: fire. He sets up a blowtorch in the lab and has everyone put their hand near it, as M'gann demonstrates that close proximity to fire reveals a White Martian in disguise. Vasquez and Demos go first and they are cleared. J'onn goes next. He initially hesitates due to his pyrophobia, but his Green Martian skin is revealed, clearing him too. Kara then orders Winn to go next. He hesitates, until he drops the charade and reveals himself as a White Martian. He knocks Kara and J'onn out and flees.
As D.E.O. agents begin a search and destroy operation, Armek sets the building's nuclear reactor into a meltdown. This is possible because when White Martians take a form of another, all their victims' properties are transferred psychically, thus Armek has Winn's know-how to overload the reactor. They also need to be at close proximity with the victim to maintain the psychic link. Alex says they have to find Winn, as he is the only one able to stop the reactor from melting down. J'onn orders Kara and Alex to the basement while he and M'gann search other floors.
In the basement Kara reveals to Alex that she was mad, when Alex bailed from her birthday. Kara says that she felt abandoned. Alex assures her that she will never abandon her, and they reconcile. J'onn and M'gann find Winn trapped in the ceiling, but to their horror they find Alex there too, indicating that there are two White Martians in the building. J'onn contacts Kara and tells her that the other White Martian has taken Alex's appearance. "Alex" then attacks Kara.
J'onn and M'gann revive Alex and Winn. J'onn takes Winn to the reactor, where Kara is fighting with "Alex". "Alex" taunts Kara for fighting her sister, but Kara is not fooled. J'onn and Winn arrive at the reactor room, where Kara is already fighting. Winn starts to work on shutting down the reactor, despite the chaos around him. Armek is also there and he engages J'onn, with M'gann coming to J'onn's aid in Green Martian form, ultimately declaring that White Martian skin is a monster. Kara is able to subdue the other White Martian, and M'gann kills Armek. Winn manages to shut down the reactor with twelve seconds to spare. As they are leaving, the other Martian raises and tries to attack, but the real Alex arrives and kills it with her alien gun she took from Maaldoria, declaring that she loves it.
The lockdown is released and Kara says she is proud of Winn. Winn then decides to go and see if James needs his help, unless Kara objects. To his surprise, Kara lets him go, and smiles happily. Alex then remembers her date with Maggie and runs. Later, M'gann and J'onn share a moment. M'gann intends to return to Mars in order to reform the White Martians, convinced that there are others like her. J'onn lets her go.
At Kara's apartment, Alex arrives with a cupcake, which is Earth Birthday-tradition for the sisters. Kara and Alex then have a conversation about how Kara felt abandoned with Alex bailing, despite all the plans she made. However, Alex believes that Kara made all those plans so that she wouldn't have to think about her feelings towards Mon-El, finally realizing she cares about Mon-El more than just being a trainer for him. The next day at CatCo, Kara and Mon-El run to each other. Kara tries to ask Mon-El for a date, but unfortunately he is already taking Eve for a date. Kara lets them go and she feels down again for losing her chance.
- The episode's title is a reference to Ray Bradbury's short story collection of the same name.
- As part of a tie-in to The LEGO Batman Movie, special LEGO end cards were shown immediately following the end credits when this episode aired on The CW.
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