Off-duty Japan Self-Defense Forces (JSDF) officer and otaku Youji Itami is on his way to attend a doujin convention in Ginza, Tokyo when a mysterious portal in the shape of a large gate suddenly appears. From this gate, supernatural creatures and warriors clad in medieval armor emerge, charging through the city, killing and destroying everything in their path. With swift actions, Youji saves as many lives as he can while the rest of the JSDF direct their efforts toward stopping the invasion. Three months after the attack, Youji has been tasked with leading a special recon team, as part of a JSDF task force, that will be sent to the world beyond the gate—now being referred to as the "Special Region." They must travel into this unknown world in order to learn more about what they are dealing with and attempt to befriend the locals in hopes of creating peaceful ties with the ruling empire. But if they fail, they face the consequence of participating in a devastating war that will engulf both sides of the gate. 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.