Supergirl goes on high alert when a new villain comes to National City. Meanwhile, Winn's new girlfriend Lyra gets him in trouble with the law and the Music Meister attacks.
Supergirl struggles to decide whether or not to obey the President's orders regarding Rhea's latest actions. Meanwhile, Cat Grant returns to National City.
Kara deals with the loss of Mon-El by focusing her energy on being Supergirl and concentrating on a mysterious new threat. Alex confesses a secret to Maggie. A citizen of National City has a mysterious connection to Kara. Lena makes a bold move.
Kara investigates a secretive new group whose leader, Thomas Coville, has a mysterious connection to Supergirl. Meanwhile, Samantha feels like she's letting Ruby down.
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 Danvers sisters closer together.
Kara investigates when an alien ship lands deep underwater beneath National City; Hank attempts to bond with his father; Samantha looks to her estranged mother for answers.