When a gate to a different world suddenly opens on Earth, Mikado City is invaded by aliens referred to as “Neighbors,” malicious beings impervious to traditional weaponry. In response to their arrival, a corporation called the Border Defense Agency has been established to combat the Neighbor menace through special weapons called “Triggers.” albeit several years have passed after the gate first opened, Neighbors are still a threat and members of Border remain on one’s guard to make sure the security of the earth .
Despite this delicate situation, members-in-training, like Osamu Mikumo, aren’t permitted to use their Triggers outside of headquarters. But when the mysterious new student in his class is dragged into a forbidden area by bullies, they’re attacked by Neighbors, and Osamu has no choice but to try to to what he believes is true . Much to his surprise, however, the transfer student Yuuma Kuga makes short work of the aliens, revealing that he’s a humanoid Neighbor in disguise.