upergirl is about Superman's cousin who, after 12 years of keeping her powers a secret on Earth, decides to finally embrace her superhuman abilities and be the hero she was always meant to be. Twelve-year-old Kara Zor-El escaped the doomed planet Krypton with her parents' help at the same time as her cousin. 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.

However, Kara's days of keeping her talents a secret are over when Hank Henshaw, head of a super-secret agency where her sister also works, enlists her to help them protect the citizens of National City from sinister threats. Though Kara will need to find a way to manage her newfound empowerment with her very human relationships, her heart soars as she takes to the skies as Supergirl to fight crime.

Overview from themoviedb.org