Approximately four years ago, a gate to another world opened in Mikado City, and invaders called Neighbors appeared.

Border sees the Neighbor Yuma as an enemy… but Osamu is unable to view him as someone with hostile intentions, and trust begins to bloom between the two boys. Osamu finds himself caught between his own thoughts and the rules of the organization he belongs to, Border.

Combat-use Trion Soldiers attack when a gate opens in Osamu’s school just outside of the Forbidden Zone. Worried he’s not strong enough, Yuma tries to stop Osamu from fighting back, but Osamu’s determination to save the other students is limitless.

Osamu gets more trouble than credit when he claims to have defeated Trion Soldier Marmod.

Kitora boldly jumps onto the back of Ilgar, a new type of Trion Soldier, while it’s mid-flight and bent on attacking the city below. Suddenly, Ilgar transforms, enters self-destruct mode and plummets toward its target. How will she escape now?!

Yuichi and Osamu's critical mission to investigate the abnormal gate puts a lot of pressure on Osamu. Meanwhile, Yuma follows Replica’s advice and conducts an investigation of his own. The next morning, Osamu and Yuichi have a fateful meeting with Yuma...