A mysterious gate suddenly appears in Ginza, Tokyo where fantasy creatures led by men in Roman armor come out from it and attack the people. Yōji Itami, a JSDF reservist, who was attending a nearby doujin convention, takes action by organizing the police to evacuate the civilians into a palace to protect them before the JSDF arrives to stop the invaders. Due to his actions, Itami is promoted to First Lieutenant and celebrated as a national hero. The appearance of the Gate and massacre of Japanese civilians, dubbed the Ginza Incident, prompts the Incumbent Prime Minister to takes action by proclaiming Japan will send a JSDF task force backed by the US to the "Special Region", the world beyond the Gate, to not only explore the new world but also capture and demand reparations from those responsible for the attack. After 3 months of preparation, the new Prime Minister sends off the JSDF task force to their mission to the Special Region with Itami as part of the task force. After finally arriving, the task force encounter an army waiting for them and prepares for battle.