Mikado City, an otherwise peaceful city, is suddenly invaded after a gate from another world opens seemingly from nowhere. Interdimensional monsters called Neighbors attack, man-made weapons prove to be ineffective against them, and the city is quickly destroyed. When all hope appears to be lost, a mysterious group known as the Border Defense Agency begins to retaliate using specialized technology called Triggers and manages to halt the invasion. Four years later, the organization continues to protect the populace of Mikado City and are now revered as heroes.

Yuuma Kuga is a new student at First Mikado City High School—a school associated with Border—which comes as a surprise, as transfers to the dangerous areas are seen as suspicious. He is soon targeted by bullies, but Border's member-in-training Osamu Mikumo stands up for Yuuma, despite his doubts about him. The two's situation turns grim when a Neighbor arrives from a gate that abruptly opens in the forbidden zone; while Osamu struggles to fend it off with his Trigger, Yuuma defeats it with ease. As Osamu inquires about Yuuma's position in Border, it turns out that Yuuma is no agent—but a Neighbor himself.