Granbelm is set in a world where magic was once commonplace but was sealed away long ago to prevent war.
Now, a magical battle royale called Granbelm takes place each month to find the one true mage, the Princeps.
A high school student, Mangetsu, unexpectedly finds herself drawn into this world and the Granbelm competition.
After being rescued by another mage, Shingetsu, Mangetsu gains her own magical Armanox and decides to help Shingetsu fight in Granbelm.
Together, they navigate the challenges and mysteries of Granbelm, each with their own reasons for wanting to participate.