“Both Sides Now” is episode thirteen of season three of Supergirl.
In the previous episode, “For Good”, the DEO learned that Julia Freeman was another Worldkiller. They are storming her house to find she’s sitting listening to music with her headphones on. Supergirl scares Julia half to death when she touches her. Julia doesn’t seem dangerous, until everyone not standing down causes her eyes to go white and the Worldkiller to emerge. She has sonic power, but J’onn is fortunately able to restrain her.
Sam finds Lena in her office. Lena reassures Sam that she thinks Sam is up to the job, reminding her how they met. But Lena wants to reduce the stress on Sam and orders her to take the day off to be with Ruby.
At the DEO, Winn says they have a Kryptonian cache crystal found at Julia’s – Purity’s – house. He might be able to track down the fortress with it. Purity’s sonic blasts are also specifically attuned to hurt Kryptonians. Alex wants to run some tests on Supergirl.
Mon-El wants J’onn’s help as the Legion ship will shortly lose the ability to fly. He thinks something from J’onn’s ship will help.
Supergirl tells Alex it was like another personality took over when Julia’s eyes went white. She doesn’t think Purity was faking it, and wants to see if they can bring Julia back out and work with her. Alex is willing to try it Supergirl’s way. And, when that doesn’t work, they’ll do it her own. They go to see Purity and, though Supergirl tries to talk to Julia, Alex is much more confrontational.
Mon-El, J’onn and Winn are with J’onn’s car/spaceship. Mon-El says one of the Legion ship’s batteries died, the one the provides anti-grav. Without power, the circuits will degrade. He wants to use the battery from J’onn’s ship to regenerate the fuel cells.
Purity is telling Supergirl that Julia is a character description. She never existed. When she and her sister arrived, they knew they’d need to hide for years before the situation was right. Supergirl talks about Julia’s roommate at university, whose life Julia saved in “Fort Rozz”. She seems to get through, but Purity reasserts herself.
Alex and Supergirl start arguing, and Supergirl wants to take it outside. Alex thinks Purity is a Worldkiller and they don’t have time for Kara’s feelings. The rest of them aren’t bulletproof.
The first attempt to fix the Legion ship doesn’t work, and it seems that Mon-El and Imra have been arguing over something.
Alex tries to get Purity to tell her where Reign is. Purity, however, starts having a go back, saying that Alex has a hole in her that she cut herself. She threw her chance of happiness away.
Alex isn’t in the best frame of mind, because of Maggie. Kara does have a point, because Purity and Julia do seem to be separate people, just as Sam and Reign are. And Sam is acting oddly again.