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.
Supergirl, with her recently drained powers taking too long to return, must deal with life without them, while DEO has a powerful escaped alien prisoner in their headquarters.