The calm days are short-lived, as the threat of another Selector Battle looms large. Kiyoi Mizushima is the first to discover that something is wrong, and she makes her move to halt the cycle of darkness. This time, the Battle contains a new card, "Key," and is set up with different rules than previously. With both the mastermind and their motivation hidden in obscurity, the darkness deepens and becomes more terrible. The Selectors, Suzuko Homura, Chinatsu Morishita, Hanna Mikage, Ruuko Kominato, Yuzuki Kurebayashi, Hitoe Uemura, and Akira Aoi, have gathered once more.