GATE 0: Thus the JSDF Fought at Ginza

In the summer of 20XX, a "gate to another world" suddenly appears in Ginza, Toukyou. What comes pouring in from it is a mysterious army and a group of hideous monsters. They go around slaughtering people, and Ginza in broad daylight quickly turns into a hellscape. While everyone is helplessly thrown into confusion in this state of emergency, there is one man brave enough to move around. That man is Itami Youji, an otaku Self-Defense Force officer who would later be called the "Hero of Nijubashi"!