Nagi is a boy working as an assistant at the Shinonome Detective Agency, which specializes in cases involving monsters and other supernatural beings hidden within the human world. The agency is led by Ren, a man who possesses a unique ability called the Fox Window, which allows him to see through the disguises of monsters and perceive their true forms. Nagi asks Ren to teach him how to use this power, leading to encounters with various yokai living secretly among humans. These include Komako, a nekomata; Jin, a crow tengu; and Hibiki, a demon. As Nagi learns to use the Fox Window, the agency receives a request to investigate a mysterious monster that has infiltrated the human world, and they must uncover its true identity. The story follows these heartfelt interactions with yokai and the mystery surrounding this unknown creature.