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.<br>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...!<br>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!!