After Sentouin D successfully infiltrated the Ranger Force in the disguise of Hibiki Sakurama, he is closer than ever to defeating the heroic Dragon Keepers. While strategizing his ideal placement in the Force, he discovers that he is assigned to the Green Squadron, whose main duty is to search and eradicate long-lost boss monsters.

Joined by another new member, Angel Usukubo; their senior, Kanon Hisui; and a mysterious informant, Chidori, Sentouin D is first tasked with identifying suspicious activity in an abandoned school frequented by delinquents. During their investigation, they fall into a trap, and the group soon realizes that a boss monster is behind it all. Sentouin D must now find a way to escape alongside the others if he wants to continue pursuing his ultimate goal—achieving his ideal world free from boss monsters and the Dragon Keepers.