Four years ago, a "Gate" to another world suddenly appeared in Mikado City, and Neighbors—invaders whom conventional weapons could not touch—attacked the city. In the midst of near-destruction, a mysterious organization called "Border" repelled the Neighbors with unknown weapons called "Triggers" and established a headquarters base. Since then, Mikado City has barely maintained peace thanks to Border. Currently, a serious boy named Osamu Mikumo attends Mikado City Third Junior High. A transfer student with white hair and red eyes, Yuma Kuga, arrives in his class. Osamu is shocked by Yuma, who is ignorant of Japanese common sense and remains uninjured even after being hit by a bicycle. After school, Yuma is taken to the old Forbidden Zone by delinquents, and Osamu follows to try and help. During this, a Gate opens behind the school—a place that should be safe—and an insect-type Trion Soldier, a Bamster, appears. Osamu activates his Trigger as a Border C-Rank agent to fight, but his training Trigger is no match, and he is cornered. Then, Yuma reveals that he is a Neighbor from beyond the Gate and activates his Black Trigger. With overwhelming power, he destroys the Bamster in an instant, saving Osamu. After the battle, Osamu accepts the fact that his life was saved by someone who should be an enemy, and he protects Yuma by lying in his report to HQ, saying "I defeated the enemy myself." Yuma also takes an interest in Osamu, recognizing his sense of justice in trying to protect others even when his strength is lacking. Thus, the two who should be incompatible—a Border agent and a humanoid Neighbor—begin to build a strange relationship of trust. Meanwhile, at HQ, an investigation into the unidentified Black Trigger reaction begins, and the upper management and A-Rank agents heighten their vigilance regarding the existence of a new humanoid Neighbor. The meeting of Osamu and Yuma marks the beginning of a destiny that will eventually lead to a great war over Mikado City and the Neighborhood. And so, it begins—

The next morning. Osamu arrived at his classroom at Mikado Third Junior High at his usual time.