An inter-world gate appeared in the middle of the Ginza shopping district of Tokyo. Through the gate came an expeditionary force from another world. Composed of Roman-like legions composed of cavalry and soldiers, supplemented with soldiers mounted on flying dragons, ogres and other monster foot soldiers they caused numerous civilian casualties. In the middle of this was an off-duty Self Defense Force soldier, 2nd Lieutenant Itami Youji, who was on his way to a convention. Taking command of the disorganized police force, he was able to save a large number of civilian lives by barricading them within the Imperial Palace. The battle ended with the utter defeat of the otherworldly expeditionary force by a squad of helicopters from Ichigaya supporting an SDF regiment from Nerima.
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 with the civilians safely evacuated in to the palace the JSDF begin to retaliate by using advanced weaponry and tactics, the JSDF easily repels the enemy capturing little more than 6,000 enemies.
The result of the failed invasion has cost lives of countless civilians in Ginza, especially the civilians who were caught in the middle of the legion's attack. As a memoir of this infamous massacre, the aftermath would be dubbed as the Ginza Incident. In response, Japanese Prime Minister had to take action by proclaiming Japan will send a JSDF task force-which mostly 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. Ironically, JSDF's expedition into the Special Region would also prompting the worlds's local residents to assume them as "invaders".