300 years after the apocalypse, Japan is suffering under the oppression of the royal family. But there is hope. Tatara, the “child of destiny” was born in the village of Byakko, and surely he will save the people. Hearing the rumors, Shuri, the youngest son of the king and also known as the Red King himself, attacks Byakko and kills Tatara. But Sarasa, Tatara's twin sister, disguises herself as Tatara and starts leading the people against the Red King, and ultimately the King of Japan. In a cruel twist of fate, Sarasa and Shuri fall in love, not realizing that the other is the hated enemy. But the truth is eventually revealed in the midst of a battle between Tatara and the Red King.
https://www.animenewsnetwork.com/encyclopedia/manga.php?id=1676