In the original anime Granbelm, magic has been extinct for nearly a thousand years, and people have largely forgotten about it. Mangetsu Kohinata, a cheery and intelligent high school girl, resides in this world, but despite how upbeat she may be about her day-to-day activities, she is unable to ignore that she is suffering from a malaise. Mangetsu possesses nothing, including nothing that makes her stand out or that she truly excels at. She doesn't excel academically or athletically, so she is all the more attracted by whatever she can actually accomplish on her own.
On a full moon night with a cloudy sky and a blood-red moon, Mangetsu leaves home once more to bring her brother's lunchbox to school when all of a sudden, everything she has ever known is upended. She finds herself suddenly outside of her modern world's school building and inside a castle complex in the heart of a city that has a medieval feel to it. But what she sees next is even more incredible: two enormous robots are engaged in combat right in front of her!
