Spoiler alert! The following recaps Supergirl Episode 313, "Both Sides Now."
In between a lot of highly charged emotional moments, there was a lot of good content tonight.
Supergirl leads a massive SWAT team into Julia's house. She is singing along with Lisa Loeb on her headphones and doesn't notice them until Supergirl taps her shoulder. She freaks out and is terrified. Supergirl tries to calm her down, but takes too long. It makes Alex antsy, and she makes a move, which makes Julia transform into Purity and release massive sonic booms that knock everyone away. J'onn eventually gets cuffs on her and they get her into a cell back at the DEO. A fortress crystal was discovered at Julia's house. Winn takes it back to the DEO to trace it. He also figures out that Julia's frequency is specifically keyed to give extra damage to Kryptonians.
Supergirl and Alex go interrogate Julia. They have wildly different styles. Supergirl handles her with kid gloves, desperately trying to reach Julia, positive she can. Alex, on the other hand, is filled with rage and wants Purity to lead them to Reign. The two women go back and forth, taking turns. Purity sees through Alex, knows that she is filled with rage because of Maggie, basically (Alex admits to this at the end of the episode).
Anyway, a lot of time is spent with the Danvers' different interrogation styles. Purity finally activates the crystal, which causes power to go out in the DEO. She uses her powers to sonic boom her way out of the prison cell, through the DEO, takes the crystal from Winn's lab, and flies out of the DEO.
Winn tracks Purity to the subway. Before the team deploys, he gives everyone earpieces that negate the effects of Purity's frequency. Supergirl knocks Purity from the train car and finds herself surrounded by Supergirl, Alex, J'onn, and Mon-El. None of them react to her sonic blast, so she does the next best thing: makes the floor collapse, sending everyone into the station below. Once there, Alex tries to evacuate while J'onn prevents the ceiling from collapsing. Supergirl and Purity face off, with Purity dropping Supergirl into a Batman-style back break. She gets Supergirl in a headlock, but Alex sees a flash of Julia trying to break through. She tries it Supergirl's way, and speaks to Purity as Julia. She is much more gentle, careful, promises that Julia is strong enough to beat Purity. It works. Julia comes out and drops Supergirl.
However, the Dark Kryptonian has sensed that another Worldkiller is ready to go, and she calls Reign to her, then advises she find Purity before Supergirl gets her. So Reign flies into the station and threatens to kill Alex (y'know, for funsies). Julia volunteers to go with Reign, and Reign takes her up on that. The two Worldkillers retreat back to Reign's fortress. The Dark Kryptonian tells Purity it is time for her awakening, but Julia doesn't want it.
Back at the DEO, Supergirl realizes that the Worldkillers might be too strong to be stopped. Instead, she wants to save them.
Elsewhere, tensions are high between Mon-El and Imra. He finally reveals to J'onn that they initially married to unite two factions. They were friends, but it was a practical marriage. Eventually Mon-El grew to love her, but being back on Earth with Kara has been a lot harder than he imagined. At J'onn's suggestion, he finally admits his complicated feelings to Imra. When asked point blank if he still loves Kara, Mon-El answers, "I don't know." She appreciates his "honesty," even though it is clear he still loves her. Regardless, Imra has something to confess to Mon-El: the real reason they are there.
When the Dark Kryptonian called Reign to the fortress, she was ice skating with Ruby. Ruby turned around and discovered her mom was gone. Frightened, she called Lena, who came and picked her up. Ruby knows something is going on with her mom, and is scared. That night, Sam returns to L Corp in a panic after getting Lena's message. Lena assures her Ruby is safe at her house, but Sam becomes furious when she finds out Lena and Ruby discussed Sam's ... "condition." Her fury almost turns her into Reign. Lena sees her eyes flash red. It only lasts a moment, then Sam comes back and she admits she just had a blackout. Lena knows what is wrong with her, and promises she can fix her.
And that's it for Supergirl until April 16! Get ready for the return of DC's Legends of Tomorrow.