In the distant past, magic was plentiful and enriched the lives of everyone until it was employed in warfare. To put an end to the resulting chaos, seven of the most powerful mages sacrificed their lives to seal away all magic. Now, nearly a thousand years later, that magical seal, known as Magiaconatus, hosts Granbelm, a monthly series of battles designed to determine which participant is worthy of the title Princeps, the one true mage. On the night of a full moon, high school student Mangetsu Kohinata is suddenly transported to an illusionary world where six mages are competing in Granbelm. Mistaken for an enemy mage, Mangetsu is attacked by two mages until she is rescued by Shingetsu Ernesta Fukami and her Armanox, Viola Katze. However, when Shingetsu is ambushed for protecting her, Magiaconatus grants Mangetsu the Armanox White Lily, enabling her to fight off Shingetsu's assailant. Overwhelmed by the revelations presented to her, Mangetsu decides to help her new friend fight in Granbelm and make her wish come true.