A diclonius named "Lucy" escapes from an experimental laboratory off the coast of Kamakura, which is being used as a facility to hold diclonius and use them for experiments. After killing many guards, she is shot in the head and falls into the sea; her brain damage causes her to form an innocent, somewhat infantile split personality, "Nyu". Later, she is found by Kohta and Yuka, two cousins who are reunited after eight years to study at university, and is taken back home with them. After she breaks Kohta's seashell, his keepsake from his younger sister Kanae who died "of an illness" eight years earlier, she runs away because Kohta gets angry with her. She does not remember anything that she has done in her entire life.