A new villain comes to National City, putting Supergirl on high alert. Meanwhile, Winn’s girlfriend, Lyra, gets Winn in trouble with the law. Maggie attempts to help Winn but old loyalties get in the way. The Music Meister attacks Supergirl.

The crossover continues on The Flash S03E17 Duet (II).

Kara and Alex head home to Midvale for a little R&R. While there, Kara recalls a painful memory of a childhood friend's death and how that traumatic experience brought the Danver sisters closer together.

Between Ben Lockwood’s new push to stir up the anti-alien movement and the Elite’s desire to target Lockwood and his minions, Supergirl is concerned about the safety of the American people – both human and alien. When Lockwood organizes a rally, the aliens decide to peacefully protest. Brainiac and J’onn join the alien march while Supergirl and Dreamer patrol to keep everyone safe. Meanwhile, James picks up his camera again to cover the march for CatCo, and Hayley assigns Alex a job that clashes with her beliefs. When The Elite and Ben Lockwood stir up trouble at the rally, Supergirl is forced to take a stand.

Kara's investigation into William Dey's "criminal" activities results in the shocking revelation he is not who she thinks. Meanwhile, the conflict between Malefic and J'onn J'onzz reaches a boiling point.

Supergirl's struggle against Leviathan reaches a boiling point as she faces off against Rama Khan. Meanwhile, as Lena and Hope work to launch Project Non Nocere, Hope proves to be an invaluable asset to Lena.

Nia and Brainy attempt to save Supergirl from the Phantom Zone by time traveling back to Kara's home in 2009. While determined to secure the item needed to help Supergirl, Brainy is worried about keeping a low profile so as not to alter the future timeline. However, when their ship crashes upon arrival, a young Kara Danvers is the first on the scene. Meanwhile, a young reporter named Cat Grant sets her sights on Midvale as she suspects there is a big story in the small town.

Part two of the flashback episodes pick up where things left off - Young Kara experiences kryptonite for the first time, Nia and Brainy are trapped by invading aliens, and a young Cat Grant may be the reason Supergirl never gets out of the Phantom Zone.

Kara lands her dream story for Catco but when the city is suddenly attacked by a nightmare monster at the same time as her interview, she is forced to take a serious look at her life and decide if she can continue to live as both Kara and Supergirl. As Dreamer takes the lead on the search for the Dream Totem, which can vanquish they nightmare monster in National City, she realizes she needs to ask her sister Maeve for help.