Mari Amachi, a devout Christian, enrolls at Perfect Religion Academy aspiring to become a nun like Mother Teresa. Her life fractures when her roommate Saori Minami is kidnapped by the Black Buddha cult, an organization corrupting the academy's religious foundation. During this crisis, Buddha appears to Mari, granting transformative powers via a sumo belt that enables her to become the magical heroine Gotaman. This form greatly enhances her physical strength, with the tightening of her loincloth directly boosting her combat capabilities. Her primary attacks weaponize her buttocks, notably the "Gladiolus" maneuver, alongside a fan weapon and slicing kicks.

Initially shy, gentle, and bashful, Mari struggles with the exposure and unconventional nature of her powers. Her heroic actions create internal conflict between her innocent personality and her role's demands. Across the OVAs, she develops a complex romantic relationship with Ryo Tobishima, the Black Buddha gang leader tasked with her elimination. This dynamic forces her to navigate conflicting loyalties—her duty to fight the cult against her feelings for Ryo. Her central friendship with Saori endures, though Saori harbors unspoken romantic feelings for her, adding emotional tension.

In the sequel, Mari confronts heightened challenges as her secret identity risks exposure to key figures, jeopardizing her mission to protect religious freedom. The culmination brings a final confrontation where her romantic entanglement with Ryo reaches a critical point, testing her resolve and forcing decisions that could end her heroic career. Her arc concludes with permanent tension between her personal desires and her responsibilities as Gotaman, leaving her future uncertain.