Supergirl does battle with the deadly Master Jailer, who is hunting and executing escaped Fort Rozz prisoners. Also, Cat hires a second assistant, Siobhan Smythe, who immediately tries to one-up Kara, and Kara and James disagree over the DEO's methods.

The Guardian debuts to lend a hand after a parasite alien drains Supergirl of her power. Mon-El considers a less than desirable new career which then leads him to contemplate his motives. Alex struggles with a new reality.

Supergirl challenges Rhea to battle to save National City. Meanwhile, Superman returns and Cat Grant offers Supergirl some sage advice.

Kara deals with the loss of Mon-El by focusing all her energy on being Supergirl and the mysterious new threat against National City. Alex confesses a secret to Maggie about their impending nuptials. A citizen of National City has a mysterious connection to Kara, and Lena makes a bold move.

Still reeling after learning about Lena’s secret, Supergirl asks Guardian for a favor that could alter his relationship with Lena forever. Meanwhile, with all three Worldkillers united, Supergirl and the team brace for an epic battle.