When a gate to another world suddenly opens on Earth, Mikado City is invaded by strange creatures known as “Neighbors,” malicious beings impervious to traditional Earth weaponry. In response to their arrival, an organization called the Border Defense Agency has been established to combat the Neighbor menace through special weapons called “Triggers.” Even though several years have passed after the gate first opened, Neighbors are still a threat and members of Border remain on guard to ensure the safety of the planet.