Thirteen-year-old Kara escaped the doomed planet Krypton with her parents' help at the same time as the infant Kal-El. Protected and raised on Earth by her foster family, the Danvers, Kara grew up in the shadow of her foster sister, Alex, and learned to conceal the phenomenal powers she shares with her famous cousin in order to keep her identity a secret. Years later, Kara was living in National City and still concealing her powers, when a plane crash threatened Alex's life and Kara took to the sky to save her. Now, Kara balances her work as a reporter for CatCo Worldwide Media with her work for the Department of Extra-Normal Operations (D.E.O.), a super-secret government organization whose mission is to keep National City and the Earth safe from sinister threats. At the D.E.O., Kara works for J'onn J'onzz, the Martian Manhunter, and alongside her foster sister, Alex, and best friend, Winn Schott. Also in Kara's life are media mogul Cat Grant, James Olsen, a photo journalist who moonlights as Guardian, a masked vigilante, Lena Luthor, and Mon-El of Daxam, whose planet was ravaged by Krypton's destruction. As Kara struggles to navigate her relationships and her burgeoning life as a reporter, her heart soars as she takes to the skies as Supergirl to fight crime.
The plot of the third season will revolve around what "means to be human", being something that will be addressed with all the characters.
James Olsen/Guardian will have a new love interest, and will return to his roots as a journalist and photographer.
Cat Grant will appear in multiple episodes in the season.
J'onn J'onzz is going to discover that he is not the last Martian on Mars and will introduce himself to his father played by Carl Lumbly, who also voiced J'onn J'onzz in the animated series of the Justice League.
Lyra Strayd will return in this season and she will appear in more episodes than the last season.
Kara deals with the loss of Mon-El by focusing all her energy on being Supergirl and the mysterious new threat against National City. Alex confesses a secret to Maggie about their impending nuptials. A citizen of National City has a mysterious connection to Kara, and Lena Luthor makes a bold move.
Psi, a thief with psychic powers, attacks National City. Able to immobilize people by tapping into their worst fears, Psi proves a formidable opponent for Supergirl. Meanwhile, James Olsen and Lena Luthor are at an impasse, and Samantha starts her new job at L-Corp.
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, and J'onn confesses an old secret.