Young detective Sonia Summers has been reassigned to the outskirts of the port city Libra after causing friction with her superiors. Libra is divided into two sectors: "the Upper," the hub of wealth and politics, and "the Outside." The Outside is rife with crime, corruption, and strange incidents involving monstrosities or youkai. Every case Sonia investigates appears to involve these mysterious creatures. Sonia encounters three unusual humanoid youkai—Bem, Bela, and Belo—who aim to protect humans and combat the evil youkai that terrorize them. Living as "Youkai-Ningen," they are ostracized by other youkai for their human-like lifestyles, and rejected and harmed by humans when they reveal their true forms. Despite the hardships they endure, these three Youkai-Ningen secretly reside among humans, continuing to pursue their dream of one day becoming human themselves.