Mikado City has been overrun by "Neighbors," formidable extraterrestrial beings that cannot be defeated by Earth's weapons. "Border," a team of soldiers equipped with enigmatic weapons called "Triggers," is the only force capable of defending against these powerful foes. Despite frequent attacks, the city has adapted to the presence of these beings. However, when a new Neighbor appears, transfer student Yuma Kuga and his classmate Osamu Mikumo find themselves in a life-or-death battle. Although Osamu uses his weapon, the alien is unbeatable. Yuma steps in and wins the fight using his own Trigger. Later it is revealed that Yuma is actually a Neighbor who has joined Border, a fact previously unknown.