Kara takes Alex home to Midvale after Alex and Maggie break up. While Alex deals with the end of her relationship, she and Kara remember the first time that they bonded as sisters while solving the murder of a fellow high school student.

Rather than wallow in self-pity, Kara and Alex go to Barry Allen and Iris West's wedding... only to find the ceremony interrupted by Nazi soldiers and villains from Earth-X including a mysteriously strong superpowered female warrior.