The story beging after a gate to another world opened in Mikado City. The entities from the other world, called Neighbors, began sending out an invasion force, called trions, that overran the area surrounding the gate.
Given their superior technology, the destruction of the city seemed only a matter of time. Suddenly an orginization called Border appeared in town, they researche the technology of the Neighbors to protect their side of the gate. In a short time they built a huge base and became established as the line of defense against the Neighbors.
One day a student named Yūma transfers to the same middle-school class as Osamu, a teenager who belongs to Border. He has a weapon called Trigger that is only allowed to be possessed by people in Border.