Mikado City, generally a tranquil place, is suddenly invaded when a gate to another dimension opens seemingly out of nowhere. Interdimensional creatures known as Neighbors launch an attack, and conventional weapons prove useless against them, leading to the city's rapid destruction. Just as hope seems lost, a mysterious organization called the Border Defense Agency begins to fight back using specialized technology known as Triggers, successfully stopping the invasion. Four years later, the agency continues to safeguard the residents of Mikado City and is now regarded as heroic. Yuuma Kuga is a new student at First Mikado City High School, an institution affiliated with Border. This transfer is surprising, as moves to dangerous zones are viewed with suspicion. He is quickly targeted by bullies, but Osamu Mikumo, a trainee at Border, defends Yuuma despite his own reservations. Their situation turns dire when a Neighbor emerges from a gate that suddenly opens in the forbidden zone; while Osamu struggles to repel it with his Trigger, Yuuma defeats it effortlessly. As Osamu questions Yuuma's role within Border, it is revealed that Yuuma is not an agent, but a Neighbor himself.