Youji Itami, an off-duty Japan Self-Defense Forces (JSDF) officer and otaku, is on his way to a doujin convention in Ginza, Tokyo, when a mysterious gateway in the appearance of a massive gate arises. Supernatural animals and warriors in medieval armor emerge through this gate, rampaging across the city, murdering and destroying everything in their path. Youji saves as many lives as he can with quick measures, while the rest of the JSDF focuses their efforts on preventing the invasion. Three months after the incident, Youji has been assigned to command a special recon team as part of a JSDF task force that will be dispatched to the world beyond the gate—now known as the "Special Region."