A gate to another world opened in Mikado City. Invaders from another dimension, referred to as Neighbors, overran the area around the gate, leaving the city gripped by fear. However, a mysterious group suddenly appeared and repelled the Neighbors.
Yuma is invited to join the Border Agency's Tamakoma Branch, and agrees on the condition that Osamu and Chika join with him. Surrounded by elite agents at the Tamakoma headquarters, Osamu can't help but feel uneasy.
With Shuji's squad singlehandedly defeated, the Border Agency's executives order Yuichi to defeat Yuma and recover the Black Trigger. Osamu worries about Yuichi's impending clash, while a secret hidden by Yuma is slowly exposed.
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.
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.