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.

[Source: Crunchyroll]