"Granbelm" is a 13-episode anime series that blends elements of magical girl and mecha genres, set in a world where magic has disappeared. The story centers on Mangetsu Kohinata, an ordinary high school girl who stumbles upon a mysterious magical artifact. This discovery draws her into the Granbelm, a secret, otherworldly battle royale where participants, known as Magiaconatus, pilot magical mechs called Armanox. The winner of the Granbelm is promised to become the "Princeps," the sole wielder of all magic in the world.

Mangetsu teams up with Shingetsu Ernesta Fukami, a skilled and determined Magiaconatus who seeks to win the Granbelm to restore her family's honor. Together, they face off against other Magiaconatus, each with their own motivations and tragic backstories. These include Anna Fugo, a ruthless and calculating fighter; Nene Rin, a cheerful but conflicted participant; and Suishou Hakamada, a cold and enigmatic figure with a deep connection to the origins of the Granbelm.

As the battles progress, the series delves into the characters' personal struggles and the moral complexities of their desires. Mangetsu, who initially lacks confidence and magical aptitude, gradually discovers her own strength and purpose. Shingetsu's resolve is tested as she confronts her past and the sacrifices required to achieve her goals. The Granbelm itself is revealed to be a cyclical event tied to the history of magic, with the true nature of the Princeps and the cost of victory becoming increasingly apparent.

The narrative explores themes of identity, ambition, and the consequences of wielding power. The battles are not merely physical confrontations but also emotional and ideological clashes, as the characters grapple with their beliefs and the impact of their choices. The series builds toward a climactic resolution that questions the very nature of the Granbelm and the sacrifices made in the pursuit of ultimate power.