Two years ago, Shibuya was ravaged by a biological terrorist attack using the deadly Ua virus. Maria Osawa was saved when her father inoculated her against the virus, but is left with partial amnesia from the shock. She works as a cameraman when she reunites with a girl named Canaan in Shanghai, China. A gifted assassin with synesthesia, she is under orders from an unknown organization for a yet unspecified mission.
A gate appears in Tokyo's Ginza district sometime in the 21st century. From the gate pours out monsters, knights from middle-age Europe, and other fantasy-like beings, and they kill many of the 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 Yōji, 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.