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 faces her first non-alien nemesis when one of her cousin's enemies--the nuclear-powered Reactron--comes looking to kill her. Meanwhile, Cat runs a derogatory exposé on Supergirl after interviewing the superhero.