In Mikado City, a peaceful town is thrust into chaos when interdimensional creatures known as 'Neighbors' launch a sudden attack through a mysterious gate. Despite humanity's conventional weapons failing, the Border Defense Agency rises to defend the city with advanced 'Triggers' technology, eventually saving the day. Fast forward four years, and the agency is hailed as heroes for safeguarding the populace. Yuuma Kuga, a new student at First Mikado City High School linked with Border, raises eyebrows due to his transfer to a risky area. Bullied initially, Yuuma finds an unlikely ally in Osamu Mikumo, a trainee member of Border who vouches for him. Their alliance faces a crucial test when a Neighbor breaches the forbidden zone, with Osamu struggling against it while Yuuma effortlessly defeats the threat. As questions arise about Yuuma's true identity within Border, a startling revelation emerges—he is not an agent but a Neighbor himself.