In the year 20XX, various Asian refugees arrive in Japan en masse to escape from their own countries over economic and political problems. To settle the refugee problem, an artificial and autonomous place called the CITY is created via land reclamation with its law enforcement handled autonomously.

To deal with criminal and terrorist activities caused by foreign residents and groups in mainland Japan, the National Police Agency created an elite anti-terrorist unit called the ACAT (Alien Organized Crime Control Department Special Investigation and Attack Team). Leading the unit is 40-year-old Mio Fuji.