The Beast Player Erin unfolds in the land of Ryoza, where the neighboring provinces of Shin-Ou and Tai-Kou have maintained an uneasy peace. Queen Shinou rules Ryoza, while her greatest general, Grand Duke Taikou, protects the kingdom with his army of powerful war-lizards known as the Touda. Although the two regions have long been bound by alliance, pressure is building fast, and the balance between them is beginning to crack under the threat of civil war.
In the village of Ake in Tai-Kou, which is responsible for raising the Grand Duke's army, lives Erin, a bright and curious girl who spends her days learning from her mother, Soyon, the village's head Touda doctor. Erin grows up watching Soyon care for the beasts that defend the kingdom, but everything changes after a disastrous incident involving the Grand Duke's strongest Touda. Soyon is punished severely, and the fragile security Erin knows is shattered. In a desperate attempt to save her mother, Erin falls into a river and is carried away toward Shin-Ou.
Unable to return home, Erin must build a life among people she does not know while trying to understand the deeper truth behind both beasts and humanity. As the tensions between Shin-Ou and Tai-Kou continue to mount, her journey becomes one of survival, discovery, and the struggle to find a place where the bond between humans and Touda can finally be understood.