The series takes place 150 years after humanity lost half of the planet to a mysterious "black fog." To contain the "black fog," walls standing 3,000 meters tall (about 10,000 feet) were erected to divide the world, and the surviving half of the human population now resides in cities known as "Arks." Shidou Enishi, one of the few Japanese survivors, operates a detective agency on the ninth ark. While chasing a male suspect involved in a burglary, he uncovers a massive conspiracy that may be linked to the Rivigarudo syndrome, which is mutating its victims into something inhuman.