A gate appears in Tokyo's Ginza district sometime in the 21st century. From the gate pour out monsters, knights from middle-age Europe and other fantasy-like beings, and they kill many citizens of Tokyo. This event is known as the Ginza Incident. The government sends a small group of soldiers from the Japanese Self-Defense Forces to the alternate world beyond the gate. Led by otaku soldier Itami Youji, they find that the villages in the world are being attacked by a dragon. An elf girl who is a survivor from the dragon's rampage joins the group in their travels across the dangerous new world.