Sonia Summers, a young detective, is transferred to the outskirts of Libra, a port city divided into two sectors: the Upper, a hub for wealth and politics, and the Outside, where crime and corruption thrive, often linked to youkai, supernatural creatures. Sonia's work involves investigating cases that involve monstrosities or youkai, with the involvement of these creatures often being a factor. She encounters three humanoid youkai, Bem, Bela, and Belo, who live as "Youkai-Ningen" among humans, excluded by other youkai and rejected by humans when they reveal their true forms. These three Youkai-Ningen fight against evil youkai and seek to become human themselves.
