Two years prior to the Hero Club's battle against the Vertex, Mebuki Kusunoki and Yumiko Miroku are among several girls who, along with Karin, are recruited by the Taisha to be trained as potential heroes. Despite her efforts to surpass Karin in every way, Mebuki becomes frustrated when Karin is chosen as the next hero instead of her. Following the Hero Club's victory over the Vertex, Mebuki and the other failed candidates are recruited as Sentinels for the Exo-barrier Special Investigations Fleet, tasked with exploring beyond the barrier and restoring the world that once was...