S4, E2: A shocking revelation causes chaos in National City. While Alex takes control at the DEO, Supergirl sets out to capture Mercy Graves (recurring guest star RHONA MITRA). Back at CatCo, Kara decides to write an investigative story on Mercy and asks Lena to introduce her as she has ties to the Luthors. Meanwhile, Brainy meets Nia and the two end up in a precarious situation.