In 2013, the city of No. 6. This is an ideal country without crime, disease, war, pain and resentment. The 12-year-old genius boy Shion (voiced by Yuki Kaji) who always believed this lived with his mother in the high-end residential area of No. 6. On a stormy night, he met someone who subverted his worldview-the scarred and violent teenager Mouse (voiced by Yoshimasa Hosoya). Shion, who was curious and fond of the mouse, not only did not feel scared but also saved the mouse. As a result, he was considered to be harboring a suspect, deprived of all privileges, and then became a park administrator. After experiencing this incident, Shion began to doubt the ideal world in which he lived, and horrible deaths frequently occurred in the city. These events are inextricably linked to No. 6. Shion, who is increasingly confused, does not know that he is on the verge of life and death. At the critical moment, he is rescued by the mouse who has not been seen for four years and comes to the slums outside No. 6. Here, Shion gradually realizes the truth about No. 6, and the danger is far from leaving them...