Ajin boys who show signs of special abilities are conscripted to serve in the imperial palace as beast-servants--status symbols and shields for their royal masters, to be kept or discarded on a whim. When they were children, Rangetsu's twin brother Sogetsu was ripped from her arms and sent to the palace to attend Prince Tenyou as a beast-servant, where he quickly fell victim to bloody dynastic intrigues. Now in a world that promises only bitterness, Rangetsu's one hope at avenging her brother is to disguise herself as a man and find a way into the palace!

Prince Tenyou's reforms have been well received by the Ajin of his country, but the rulers of the neighboring kingdoms are anything but pleased. With war brewing on the borders, Rangetsu and the prince travel to a market town to seek information from a mysterious merchant. Will their reconnaissance efforts be for nothing when battle breaks out?