Introduction Arc

The Introduction Arc is the first arc of the series.

Characters

 * Osamu Mikumo
 * Yūma Kuga
 * Replica
 * Yūichi Jin
 * Chika Amatori
 * 3 Idiots
 * Moribayashi
 * Mizunuma
 * Ichinose
 * Mizunuma
 * Futatsugi
 * Miyoshi
 * Yotsuya
 * Shūji Miwa
 * Yōsuke Yoneya
 * Jun Arashiyama
 * Ai Kitora
 * Mitsuru Tokieda
 * Kyōkō Sawamura
 * Katsumi Karasawa
 * Eizō Netsuki
 * Masafumi Shinoda
 * Motokichi Kinuta
 * Takumi Rindō
 * Masamune Kido
 * Shun Midorikawa
 * Takaaki Katagiri
 * Karin Yuitsuka
 * Aoba Harukawa
 * Shōhei Kodera
 * Tōru Narasaka

Trion Soldiers

 * Replica
 * Bamster
 * Marmod
 * Ilgar
 * Bander
 * Rad

Triggers

 * Raygust
 * Scorpion
 * Asteroid
 * Kogetsu
 * Bagworm

Neighbor Attack
In Mikado City, monsters known as Neighbors began appearing through gates and attacking the city. Nothing seemed to be able to defeat them, and many people died or went missing during the invasion. Suddenly, an organization defeated the Neighbors using Neighbor technology called Triggers. The organization, called Border, was then planted in Mikado City with its primary goal to defend the city against Neighbors.

A Meeting of Fate
15-year-old Osamu Mikumo is sitting in class when he hears of a transfer student coming to his school, which piques his interest. Meanwhile, Yūma Kuga, a student who will be studying in Mikado City, is hit by a car, but is mysteriously uninjured. As he went to the Border headquarters before school, he arrives late. While he introduces himself to the class, a student points out that he's wearing a ring, which goes against the school dress code. A teacher informs him that he cannot wear his ring at school, but Yūma states that it's a memento of his father. Being intimidated by Yūma's eyes, he reluctantly allows him to wear it. Another teacher there assigns Osamu as his guide. During reading time, a bully throws several paper balls at the back of Yūma's head, telling Yūma it's a Japanese greeting. Yūma appears to understand, and squishes the paper even harder, and flicks it back at the bully, knocking him off his chair.

After school, Osamu chides him for striking back, saying it's best not to do anything. Yūma finds this surprising, stating that where he comes from, if one didn't strike back, they would be attacked further. The bullies then reappear in a larger gang, forcing Yūma to come with them. Osamu objects, prompting the bullies to take him along with them and lead the two to a Neighbor-prone zone. During a brief scuffle, Yūma breaks one of the bullies' feet. The others proceed to attack him, when a gate opens and a Bamster Neighbor appears. All the bullies manage to escape, except for the one with the broken foot. Osamu rushes in to save him, changing into a Border uniform, revealing his status as a Border agent. He attempts to attack the monster, but cannot break through its skin with his sword Trigger. Yūma joins the fray, revealing that he possesses a Trigger, in the form of his ring. He easily defeats the Neighbor.

After the battle, Osamu asks Yūma about his Trigger, with Yūma stating it was his father's. He also reveals that his father wasn't part of Border. Osamu expresses surprise, since only Border Agents are allowed to have a Trigger. Yūma then reveals that he is a Neighbor.

Japanese Customs
A team arrives at the scene where the Neighbor attacked. Someone tells one of them that there were no teams in the area, leaving him to wonder who could have done this.

Meanwhile, Osamu questions Yūma about his Neighbor status, and Yūma then says to forget everything he said. Yūma holds a large amount of money and asks Osamu if he could buy anything with it, attracting the attention of people around. Two men appear Yūma demanding money for a "broken leg". Osamu begins to tell Yūma that they were lying, when they assault him. They now demand the money from Yūma, who proceeds to break one of the gangster's leg, deeming it a trade-off, and giving them the money.

Osamu goes off to buy food when another gang approaches Yūma. A member of this group states that his father was ill, and needed the money. Yūma said that he would, but if they were lying, he would attack them. They agree, but Yūma's Side Effect determines they are lying and attacks them. Osamu chides Yūma once again for being too aggressive and tells him that he would teach him Japanese customs. Yūma thanks Osamu and begins to walk across the road during a red light, only to be hit by a car. Due to his Trion Body he survives the impact.

Pre-Second Neighbor Attack
Osamu and Yuma talk about keeping Yuma safe by saying he shouldn't use his trigger, Replica whispers to Yuma and says he agrees with Osamu's reasoning, while they eat lunch on the roof of the school. The three idiot bullies appear to interrupt everyone's lunch, ordering the freshmen to give them a Roof Usage Fee, to which Osamu jumps and tells them off only to be hit and mocked. Yuma is confused by their actions, and Osamu tells him that Border erases people's memories when they encounter Neighbors. Knowing he shouldn't attack the bullies, Yuma stamps the roof hard enough to scare and send a shock wave up the broken leg of the leader of the bullies, which makes them leave in fear.

Back in class, people crowd around Yuma complimenting his actions and saying he's awesome, while he tries to tell them that he is average. Eventually, they ask where he was from, and he says that he's moved to many countries, and since he could remember he has only known war. The students can't imagine growing up in war, and tell Yuma about the first attack of the Neighbors four and a half years ago. Over 1,200 people died and over 400 are still missing. Yuma once again is confused about how everyone handles this so calmly, before being interrupted by Miyoshi who proclaims Border the greatest thing in the world who stop Neighbor attacks.

After scurrying Yuma away from the crowd of students, and before he could reveal Osamu's Border status, Osamu tells Yuma that he has to stay a secret from everybody, but then the alarm goes off and two Marmods attacks the school. Most everyone escapes, but Osamu learns that one of the Marmods is cornering students, but before he could go off to rescue them Yuma stops him and says that he will die if he goes. Osamu recognizes that, but says that's not who he is and runs off anyways. He finds the Marmod and holds it off as the students rush to safety. After being cut, slashed, and stabbed, Osamu reverts back to his human form awaiting the lethal strike, but Yuma comes to the rescue! To mask his presence from Border, Yuma uses Osamu's trigger and easily defeats the two Marmods. Osamu is stunned that Yuma could defeat two Neighbors with only his C-rank training Trigger. At this Replica reveals himself to Osamu and explains Trion.

In every human, there is an organ, invisible, next to the heart that carries Trion. Like abilities and reflexes being different from each human so is Trion. Yuma has more Trion than Osamu, therefore he could create a better, sturdier weapon and could easily defeat the Marmods than Osamu.

To make Osamu believable as being the hero, Yuma plays a victim and lets Osamu carry him out of the rubble to a cheering crowd. With everyone around them, Yuma starts to tell the heroic story of how Osamu slew the Neighbors, to which people cheered. At this time Arashiyama squad finally arrives on the scene and inspects the dead Neighbors. They are surprised to see a C-rank Border agent defeating them, and question whether Osamu did a good job, or should be punished for using his trigger (trainees are not supposed to use triggers outside of Border). Jun, the leader of Arashiyama squad, congratulates him because his siblings were at the school and he thought he made the right call; however, Ai Kitora thinks he should be punished for saving everyone and using his Trigger that Border owns. To that, Yuma calls her out saying Triggers were stolen from the Neighbors, asking if they ask the Neighbors whenever they used Triggers, and basically embarrassing her in front of everyone because she didn't like Osamu getting attention. Ending the conversation, Jun finally states that whether Osamu did the right or wrong thing, it would be up to Border to decide.

Second Neighbor Attack
Ai Kitora meets Osamu and Yuma after school to escort Osamu to Border for judgment. Yuma and Kitora get into an argument while a portal opens up revealing an Ilgar flying toward the city ready to unleash bombs upon it. Kitora tells Osamu to stay, he has no more trion to transform, and states that she will take it down herself. Osamu can't stay behind, and with Yuma telling him he couldn't take the Ilgar down he states that he can help the people on the streets. Meanwhile, Kitora jumps on the Ilgar's back and starts attacking it, Yuma looks on as she activates the Ilgar's self-defense bombs on its back. Eventually, she is able to wound it deep enough that it starts to fall; however, it falls in the direction of the worn-torn city it was bombing. Kitora tries, but can't do anything to stop it. Yuma activates his trigger, and with Replica telling him that Osamu said not to be noticed, he attaches a chain and pulls the Ilgar into the river where it then safely explodes. Kitora knows someone helped her take down the Ilgar, and she looks on with hate toward Osamu who helped the civilians. Osamu gave all the credit to Kitora as the one who saved them, and Yuma tells Kitora that there is no contest who the better agent is, Osamu looks beyond rules and ranks. Somewhere else, Yuichi Jin sits upon a pile of Ilgar's that he took down, and he is called into Border.