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! After years of pursuing vengeance, Rangetsu has finally found news of her brother, and he's alive!

But Sogetsu is locked up in the inner palace, where his very flesh is being harvested as medicine for an emperor obsessed with immortality. And now that the emperor has his hands on both twins and their mysterious powers, he's never going to let them go...