The prince’s cherished butterfly, Reirin, basks in royal favor, but the envious court schemer Keigetsu—dubbed the “sewer rat”—casts a spell that swaps their bodies and fates. As Reirin’s execution looms, she finds unexpected liberation in Keigetsu’s robust form, while Keigetsu, trapped in Reirin’s delicate shell, discovers that the beloved’s life is a gilded cage—and soon gets ensnared in a dangerous conspiracy festering within the palace walls.