Monsters suddenly appeared in cities, attacking humans one after another and quickly becoming a threat to humanity. Recognizing the problem, the government established an organization called "MSS" with the purpose of exterminating these monsters. Kisaragi Itsuki, who overcame rigorous tests and harsh training to join "MSS," an elite unit for monster extermination, finds himself fighting monsters from his very first day of enlistment...! While carrying the desire to "find and save his brother" and the huge secret that "he himself is one of the monsters," he challenges the monsters head-on!!