When a portal to another world suddenly opens on Earth, Mikado City is invaded by strange creatures known as "Neighbors," malicious beings that are impervious to traditional weaponry. In response to their arrival, an organization called the Border Defense Agency has been established to combat the Neighbor menace using special weapons called "Triggers." Even though several years have passed since the gate first opened, Neighbors remain a threat, and members of Border stay on guard to ensure the safety of the planet. Despite this delicate situation, trainees such as Osamu Mikumo are not 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 and attacked by Neighbors, Osamu has no choice but to do what he believes is right. Much to his surprise, however, the transfer student Yuuma Kuga quickly dispatches the aliens, revealing that he is a humanoid Neighbor in disguise.