Supergirl’s powers are tested when Reactron, one of Superman’s formidable enemies, arrives in National City and targets the young hero. Also, Cat plans to run an exposé on Supergirl.

Personal and professional stress get the better of Kara when she goes too far during a training exercise against Red Tornado, a military cyborg commissioned by Lucy Lane’s father, General Sam Lane. Also, Cat’s tough exterior is shaken by a visit from her judgmental mother, Katherine, and Alex enlists Winn to look into her father’s mysterious death.

With Astra in captivity at the DEO, her husband, Non, captures Hank, leading to a tense standoff between the two sides. Also, Kara continues to refute Cat's allegations that she is Supergirl.

Kara faces off against her mirror image when Bizarro, a twisted version of Supergirl, sets out to destroy her. Also, Kara grows closer with Cat’s son, Adam.

Kara travels to Superman’s Fortress of Solitude in hopes of learning how to defeat Indigo, a dangerous being who can transport via the Internet and who has a connection to Kara’s past. Also, James’s relationship with Lucy reaches a crossroads.

Kara gains a new ally when the lightning-fast superhero The Flash suddenly appears from an alternate universe and helps Kara battle Siobhan, aka Silver Banshee, and Livewire in exchange for her help in finding a way to return him home.