From Crunchyroll:
On August 20XX, a gate suddenly appears in Tokyo's Ginza district, unleashing a portal where monsters, medieval knights and other fantasy beings come from another world and wreak havoc on Tokyo. The Japanese Defense Force take action against these monsters and push them back into the "Gate". Third Reconnaissance Team is dispatched to the "Special Region" lead by officer (and otaku) Touji Itami.

Note: The volumes are paired and the chapter numbering resets every 2 volumes.