Cast member Chyler Leigh makes her directorial debut in the new "Prom Again!," a flashback episode that sees young Kara (guest star Izabela Vidovic) experiencing exposure to kryptonite for the first time. Meanwhile, the time-traveling Nia and Brainy (Nicole Maines, Jesse Rath) find themselves trapped by aliens, and a young Cat Grant (guest star Eliza Helm) poses a threat to Supergirl ever getting out of the Phantom Zone again.