Kara is moved by the story of a missing woman named Izzy and decides to investigate despite Snapper Carr's order to leave it alone. Kara takes Mon-El with her to the last place Izzy was seen and the duo find themselves thrust through a portal to another planet, Slaver's Moon, where the downtrodden are sold as slaves. The leader of the trafficking ring is none other than Roulette. To make matters worse, Slaver's Moon has a red sun, which means Kara and Mon-El are stripped of their powers and stuck on the planet with no way home. Meanwhile, back on Earth, Alex blames herself for Kara's disappearance, fearing she's become too focused on her new relationship with Maggie and too complacent in watching out for her sister.

Kara comes face to face with Red Daughter and the two engage in an epic battle. Determined to find a way to extract the Harun-El, Lena turns to an unlikely source for help - Lillian Luthor. Meanwhile, Alex realizes she's missing parts of her memory, and Brainy, Nia and J'onn set out to track down aliens who have been abducted by Lex.