"Both Sides Now" begins with Kara and the DEO converging on the home of an alleged Worldkiller. Winn shares that they have one heat signature on the inside, and when Kara and Alex break down the door to enter the home, they find Julia Freeman — the lady we previously saw that was hit by a car — in a back room.
Julia has no idea why Supergirl and the DEO are in her home, and when a startled Alex pulls her gun on Julia, the latter unleashes her powers against the team. Julia exhibits a sonic scream — much like Black Canary or Black Siren — but J'onn is eventually able to subdue her.
Lena and Sam are talking at CatCo about Sam's recent job performance. Lena shares that she's looking for additional help to help take over Sam's role, ordering the yet-to-be-revealed Worldkiller to take the day off.
The team assembles at the DEO, attempting to find the home base for the Worldkillers. Mon-El, J'onn, and Winn leave Kara and Alex alone. Kara admits to her older sister that she's bothered by the fact that it seems that someone took over Julia's body.
Elsewhere, Mon-El asks J'onn and Winn for their help in fixing the Legion ship so he and the rest of the Legion can go back to their respective time.
Kara and Alex walk into Julia's holding cell to begin an interrogation. Julia's eyes are still white, hinting that the Worldkiller persona is still active. Julia reveals her Worldkiller name is Purity.
While Kara is trying to play "good cop," Alex steps in to play the opposite role, threatening to never let Julia out.
Sam picks Ruby up from school so the two can both play hooky and go ice skating.
J'onn takes Mon-El and Winn to look at his own ship, which poses as an undercover convertible. Mon-El tells J'onn that the Legion ship has lost power, and J'onn offers the power source from his own ship to help out.
At the DEO, Kara tries to get Purity to switch back to Julia. Kara uses a picture of one of Julia's friends from the past in hopes of luring Julia back, but the Worldkiller persona doesn't budge. Purity threatens to beat Kara to a pulp, and a disgruntled Alex pulls Kara outside, where the two fight about their vastly different treatment of the Worldkiller.
The trio is working on getting the battery to work on the Legion ship. Imra walks in, trying to help, and a frustrated Mon-El gets in a tense moment with her, and it's revealed the two are at a rocky point in their relationship.
Back at the DEO, Purity gets into Alex's head, who storms out of the room upset. Kara confronts the Worldkiller about Alex being right.
Sam and Ruby are seen ice skating.
Winn's working in the lab when he notices an upset Alex pacing back and forth outside. Winn notices the power source from the Legion ship he's working on has turned on in the lab, and goes to investigate.
Sam and Ruby are in the middle of a race at the ice rink when Sam's eyes turn a different color. Ruby turns around to talk to her mom, and Sam is nowhere to be found.
Purity uses her powers to break out of her holding cell and knock Kara unconscious. She confronts Winn and takes the power source from him before escaping from the DEO.
J'onn returns to the Legion ship with a bottle of "Martian moonshine" to share with Mon-El. The Martian Manhunter informs Mon-El that he was married once upon a time. Mon-El opens up and reveals that his marriage to Imra was essentially a pretend marriage.
Mon-El admits to J'onn that he limits his time around Kara now because he still has feelings for her, and Imra gets upset. J'onn says that marriage is "sharing," and encourages Mon-El to share his feelings with Imra so she knows what's going on. J'onn's phone goes off, and the pair is alerted to Purity's breakout at the DEO.
Lena approaches a distraught Ruby at the skating rink. Lena reassures Ruby that she'll get to the bottom of her mom's recent antics.
J'onn and Mon-El arrive at the DEO the same moment Kara and Alex walk in. Winn gives the four of them earpieces that would block Purity's sonic powers. Reign is at her Fortress and receives an order to find Purity and bring her back there.
Purity's starting to do some sort of ritual on the subway, and is soon approached by Kara, and the two begin fighting in the subway tunnel. Mon-El, Alex, and J'onn surround her, and although the team is unphased by her powers thanks
The group begins fighting in the subway station, where dozens of civilians run around frantically. Kara and Purity fight while J'onn, Winn, and Alex focus on rescuing all of the civilians from the damage caused.
Purity has overpowered Kara until Alex runs up and starts addressing her by Julia. Julia takes over her body once more and lets Kara go. Reign flies in and is about to kill Alex when Julia offers herself up. Reign takes Julia and flies away.
Reign has taken Julia back to the Fortress of Sanctuary, where she's presumably going to begin some sort of training.
On the Legion ship, Winn has fixed the power source and leaves Mon-El and Imra alone. Mon-El apologizes to his wife for the way he's been acting and admits that being back by Kara is confusing for him. Imra asks her husband if he still loves Kara, and he admits he doesn't know. Mon-El says he's willing to work through it, but Imra distances herself and tells her husband that it's finally time she tells him why the Legion is really in National City.
Kara and Alex are out having a drink. The two talk about their troubles with love and reassure each other that they'll eventually find "the one."
Sam runs into Lena's office looking for Ruby. Lena reveals that Ruby informed her of the other times Sam has run away. An upset Sam blacks out again, and Lena realizes what Sam's issue is. Lena reassures Sam that she's going to make it all better.