As the weight of his innumerable failures in life take their toll, Etsurou Kurosu comes to a sole solution—take his own life. Tying a belt around a wall hanger and setting it around his neck, he lets the noose do its work. However, he panics and his flailing creates a hole in his wall. Through this hole he sees his beautiful neighbor Rio Miyaichi, and peeping at her daily life soon becomes his obsession.

However, during one of his spying sessions, Kurosu witnesses Rio's shocking hobby—murder. Though she realizes that he has seen too much, Rio does not kill him. Instead, she offers him a simple proposal: accept her for who she is. Agreeing to this, Kurosu begins dating her. While Rio continues her systematic murders, she starts to involve him in her ambitious plans as well. Now dragged into the seedy underworld of killers, Kurosu not only struggles to stay alive, but to also understand his murderous new girlfriend.