Gate

Youji Itami, a Japan Self-Defense Forces officer and anime fan, is heading to a doujin convention in Ginza when a massive, mysterious gate appears in the city. Out of it pour medieval warriors and supernatural creatures, who begin slaughtering people and wreaking havoc. Youji springs into action, rescuing civilians while the JSDF mobilizes to halt the invasion. Three months later, Youji is assigned to lead a reconnaissance team into the world beyond the gate-now called the "Special Region." Their mission is to gather intel, explore this strange land, and establish diplomatic relations with its ruling empire in hopes of preventing further conflict.