One day, a gate to a different world opened in Mikado City. Invaders from another dimension, mentioned thereafter as Neighbors, overran the world round the gate, leaving the town gripped by fear. However, a mysterious group suddenly appeared and repelled the Neighbors. This Border Defense Agency independently researches Neighbor technology and fights to guard this world. during a short time, they managed to create a huge base and prepare a defensive system against the Neighbors. it has been four years since then.