One day, a gate to another world opened in Mikado City. Invaders from another dimension, referred to thereafter as Neighbors, overran the area around the gate, leaving the city gripped by fear. However, a mysterious group suddenly appeared and repelled the Neighbors. This Border Defense Agency independently researches Neighbor technology and fights to protect this world. In a short time, they managed to build an enormous base and prepare a defensive system against the Neighbors. it's been four years since then.
Junior high student Osamu Mikumo is one of Border's members. One day, a transfer student comes to his class. For some reason, this boy named Yuma Kuga has a weapon called a Trigger, which only Border personnel are permitted to carry. When Osamu questions him about it, Yuma says, "I came fro the world beyond the gate. I'm one of what you guys call Neighbors."
The story of Yuma and Osamu is set into motion.