As the burden of his countless life failures weighs on him, Etsurou Kurosu arrives at a single solution: to end his own life. He ties a belt to a wall hook and places it around his neck, allowing the noose to take effect. However, he panics, and his flailing creates a hole in his wall. Through this opening, he sees his beautiful neighbor, Rio Miyaichi, and watching her daily life soon becomes his obsession. During one of his spying sessions, however, Kurosu witnesses Rio's shocking hobby: murder. Although she realizes he has seen too much, Rio does not kill him. Instead, she makes him a simple proposal: accept her for who she is. Kurosu agrees and begins dating her. While Rio continues her systematic murders, she starts involving him in her ambitious plans as well. Now dragged into the seedy underworld of killers, Kurosu not only struggles to stay alive but also to understand his murderous new girlfriend.