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]

Border Defence Agency was established to counter the attacks from the "Neighbors," beings from the other world possessing unknown forces. A low rank Border agent, Osamu Mikumo, teams up with a Neighbor, Yuma Kuga, and a childhood friend, Chika Amatori. They strive to win through the Rank Wars within the Border to be selected for the away squads to the "Neighborhood," the other world. Meanwhile, another attack from the Neighborhood was detected. Mikado City is still damaged from the second large-scale invasion by the largest military nation, Aftokrator. To prevent panic among the city and citizens, Border arranges a confidential interception based on the foresight of Yuichi Jin by mainly deploying selected A-Rank elite squads. But when a Gate appears, and Attacker rank #1 Kei Tachikawa set to take action, Jin sees an ominous future.

[Source: Toei Animation US]