Armek (guest star Terrell Tilford), a White Martian, descends on National City intent on taking M'gann (guest star Sharon Leal) back to Mars to face her punishment as a traitor. Hank (David Harewood) and Supergirl (Melissa Benoist) determine the best way to keep M'gann safe is to bring her to the DEO.
However, when it is revealed that Armek shape-shifted into M'gann and is now loose in the building, the team realize he could be disguised as any one of them.