Twenty-four-year-old Kara Zor-El, cousin of Superman, who was taken in by the Danvers family when she was 12 after being sent away from Krypton, must learn to embrace her powers after previously hiding them.
Supergirl's powers are put to the test when one of Superman's most formidable enemies, Reactron, shows up in National City to take her down. In the meantime, Cat makes plans to run an expose on Supergirl.
Stress makes Kara go too far during a training exercise against a military cyborg commissioned by General Sam Lane, while Cat's mother comes to visit and Winn helps Alex investigate her father's death.
Kara does her best to support Winn when he hears that his father Winslow Schott Sr. breaks out of prison. His father looks for Winn for a reason to become like him. Cat offers Lucy a job working at CatCo and Alex asks Hank to use his powers to find out and uncover Maxwell Lord's plans.
Kara helps Hank face his painful past when a White Martian, who wiped out all of the Green Martians from Mars, kidnaps Senator Miranda Crane who is an anti-alien politician. Adam Foster who is Cat's estranged son arrives in National City.
Kara travels to the Fortress of Solitude hoping to find information on how to defeat Indigo, a being from her past who can travel through the Internet. In the meantime, James and Lucy reach a crossroads in their relationship.