Yūma and Replica continue to tell Jin and Osamu about Yūma's past and Calvaria's against Spinthir, explaining how Yūma's father Yūgo sacrificed his life to create Yūma's Black Trigger. Afterwards, Replica asks Osamu to give Yūma a new goal, and then Shiori then tells Osamu about Chika's decision to join Border, and Osamu and Chika discuss it. Osamu then decides to invite Yūma to form a team with him and Chika, seeing her determination, and Yūma accepts with the condition that Osamu is the team captain. They then go back to Mr. Rindō's office to file their enrollment, where they find out Jin foresaw that.
Flashing back to the past in the Neighbor world, Yūgo and Yūma (with black hair) helped Raymond, the leader of their country's defense and Yūgo's long-time friend. When the fortress was attacked, Raymond and Yūgo told Yūma to stay out of the battle, since the attackers most likely had a Black Trigger. Spinthir forces suddenly attacked, and the Calvarian Trigger controllers were outnumbered. Yūma decided to join the battle, disregarding his father's orders. A strange Neighbor with claws appeared and attacked Yūma, killing him. However, Yūgo created a Black Trigger, sealing off Yūma's dying body inside and replacing it with a Trion body. When Yūma woke up, his hair was white and he watched his father's body disintegrate, having used all his power to save his son.
The military pressured Yūma into using the Black Trigger to seek vengeance for Yūgo's death, not knowing that Yūma also inherited his father's Side Effect, detecting lies. Hearing this, Osamu suddenly realizes that Yūma really could tell if people were lying previously, but thinks this power must be a difficult one to have. After his father's death, Yūma decided to finish the fight the two of them started, until the war ended in reconciliation. Replica suggested that Yūma find Border and his father's friend, hoping that someone there could help Yūma get his old body back.
Replica explains to Osamu that Yūma has not grown since he was 11 years old, because he has a body made completely of Trion. Inside the Black Trigger, Yūma's original body is still slowly dying, and once it completely dies, Yūma's Trion body will also die. While Replica wanted someone to help Yūma gain his original body, Yūma actually hoped to find a way to resurrect his father. Hearing the news about Mogami Soichi, Yūma no longer has any hopes that someone from Border can help him and no reason to live. Replica implores Osamu to help Yūma find a new purpose. Yūma admits that he may no longer have a purpose in life, but the last few days have been fun. Jin assures Yūma that there will be more fun things in the future.
As Osamu ponders on helping Yūma find a purpose, he learns that Chika has decided to join Border in order to look for her brother and friend who were abducted. Osamu attempts to dissuade Chika in order to keep his promise to Rinji. Chika understands the near impossibility of finding her brother, but is surprisingly persistent in her desire to join. Shiori explains that there is currently no room for Chika to join Tamakoma's team, and she should go to HQ in order to create her own team.
Osamu relates Chika's decision to Yūma and their goal of creating a team, joining Tamakoma Branch, and aiming for A-Rank. He then asks Yūma to join them, who still refuses. Osamu implores Yūma to help them achieve A-Rank and selection for expeditions to the Neighbor world. Yūma surprises Osamu by noting the similarity between Osamu and Yūgo, because he smiled as he died to save Yūma. He asks Osamu why he fights, even risking his own life, for others. Osamu explains that if he runs even one time, he feels he may run when he really needs to stay and fight. Yūma stands up and declares he will help them out. He says that joining their team sounds like fun, but he will only join on the condition that Osamu is the leader. Chika agrees, and they head to Rindō's office. There they find the applications already prepared. Yūma asks Jin if he saw this with his Side Effect, and Jin reminds him that he said there would be more fun things in the future. Rindō officially welcomes the team to Tamakoma Branch as they prepare to aim for A-Rank.