In the land of Ryoza, the neighboring provinces of Shin-Ou and Tai-Kou have long coexisted in peace. Queen Shinou rules over Ryoza, while her foremost general, Grand Duke Taikou, defends the kingdom with an army of mighty war-lizards known as the "Touda." Despite the enduring alliance between the two regions, mounting friction threatens to ignite a brutal civil war.
In Ake, a village within Tai-Kou tasked with raising the Grand Duke's army, lives Erin, a cheerful girl who spends her days observing the work of her mother Soyon, the village's head Touda doctor. However, while under Soyon's care, a catastrophic incident befalls the Grand Duke's most powerful Touda, and the peaceful life Erin and her mother had known vanishes as Soyon faces severe punishment. In a desperate attempt to save her mother, Erin falls into a river and is swept away toward Shin-Ou.
Unable to return home, Erin must learn to build a new life among completely different people, all while seeking the truth about both beasts and humanity itself, as tensions between the two regions continue to escalate.