Story Arcs

Yūma Kuga transfers to Mikado City's third junior high school, where he meets Osamu Mikumo. Yūma learns of Border and the Neighbor invasions, and reveals that he himself is a Neighbor.

Yūma's identity as a Neighbor is outed to Border, who sends their best units to assassinate him and take his Black Trigger. However, he was recruited into Tamakoma Branch by Yūichi Jin, who joins force with Arashiyama Unit to protect his new junior.

On Border's Official Enlistment Day, Yūma and Chika Amatori finally make their debuts, while Osamu battles Sōya Kazama, calling attention to their group. Later, Osamu and Yūma battle Shun Midorikawa, and are afterwards called to a meeting regarding an upcoming invasion.

A group of humanoid Neighbors, later identified as being from Aftokrator, invade Mikado City in search of Trigger users. Based on Jin's predictions, Border fights to protect civilians and C-Rank trainees.

After the invasion, Yūma and Chika officially become B-Rank agents, and form Tamakoma Second with Osamu as the captain and Shiori Usami as the operator. The unit joins the Rank Battles in order to reach A-Rank and join the Expedition Force. Meanwhile, Border's A-Rank units and free B-Rank units thwart an expedition from Galopoula and Rodochroun to destroy Border's Expedition Ship. Out of necessity for the team, Hyuse joins Tamakoma Second.

Tamakoma Second attended the training camp in Shizuka City. They met Xeno, a genius Trion Soldier engineer, along with Lilith, a Trion Soldier, who were running away from their planet - Ergates. Gieve and Charon were chasing after them in order to retrieve Lilith to prevent information on Trion Soldier technology from getting leaked. Tamakoma Second cooperated with Xeno and Lilith to stave off Gieve and Charon's attacks while waiting for their opportunity to leave Earth.