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!

Rangetsu isn't the only person in the imperial palace with dark secrets. Prince Kougai's beast-servant Boku knows something about Sogetsu's final hours, and his confession shatters everything Rangestu thought she knew about her brother's death!