"Period" is the term used for magic users who defeat the "Spiral," monsters summoned from isolation. As these creatures become more prevalent, 14-year-old apprentice Period Haru, belonging to the Eighth Arc-end Division, is summoned to join the endless battle and put a stop to it. Sadly, a mysterious theft devastates their division, leaving Haru and his comrades with no choice but to leave their headquarters. To restore their branch, they must complete one quest after another.