On May 7, 1615, in the early Edo period, Sanada Yukimura, hailed as “Japan's greatest warrior,” met a tragic end after being defeated by the Tokugawa forces at the Osaka Summer Siege. With the fall of Osaka Castle, the Toyotomi clan was annihilated.
Meanwhile, among the Date forces fighting for the Tokugawa, the warrior Katakura Shigenaga (Kojuro) was feared as the Demon Kojuro for his fierce fighting. The day before, on the 6th, at the Battle of Domyoji, he had distinguished himself by slaying Gotō Matabee, a mighty warrior of the Toyotomi faction.
Thus, just before the final battle, Yukimura, concerned for the future of his children, decided to entrust them to Katakura Kojuro—an enemy, yet one he recognized as a brave and formidable warrior. After the battle, moved by Yukimura's sense of honor, Kojuro took the children (including Oume and Daihachi) and two dogs back to Shiroishi Castle in the Sendai domain, ruled by Date Masamune, to raise them. The scene then shifts abruptly to a classroom at Hakuhou High School, where Takahashi Asuka attends.

At Hakuhou High School, alongside regular classes, students are organized into cross-grade-level groups. They engage in research projects addressing issues like SDGs, disasters, the environment, and regional revitalization, fostering connections with society. Asuka happened to join the same history group as her classmate Yusuke, but they argued over a minor issue. Feeling downcast, she set off alone with heavy steps toward “Kamiishi Shiroishi” near Shiroishi Castle.
After arriving, Asuka prayed and began exploring the area. Suddenly, she felt darkness closing in around her feet. Looking up, she saw the clear blue sky rapidly being swallowed by thick, leaden clouds, as if in fast-forward. ・・・・

-----　This story is a work of fiction incorporating elements of historical fact.　-----