The anime takes place in a world that long ago featured the existence of magic, but has long since lost that ability. The story begins when the very normal high school student Mangetsu Kohinata meets Shingetsu Ernesta Fukami, who has migrated back to Japan from Germany, on a night with a full moon.
The night of the crimson full moon. The full moon of Konichi, who had been living a normal high school life, is caught up in the battle to become the only magician in the world at the school where he returned to get his forgotten. It is explained that this world was overflowing with magic from the girl and the new moon which met there, and it is a full moon which runs away puzzled.
As dawn breaks, Mangetsu awakens in Shingetsu's apartment. On her way back home, Shingetsu tells her more about mages and GRANBELM. Determining that Mangetsu has no intention of fighting, Shingetsu prepares to erase her memories, but her technique is unsuccessful. When Shingetsu is about to give up, Mangetsu asks her for more information about GRANBELM.
Mangetsu meets Anna at school in a chance encounter, and Anna tells her that Shingetsu ruined her family. During lunch the next day, Mangetsu tells Shingetsu that she wants to participate in GRANBELM. Shingetsu offers to help her, but at that moment, a car carrying Anna and some of her family passes by
Mangetsu experiences her second GRANBELM. As the battle grows more intense with all the girls' emotions running high, Mangetsu and Shingetsu struggle against Nene's ARMANOX, JI GUAN LONG. But all along, another mech is approaching them
Nene reveals her identity to Mangetsu and Shingetsu, vowing to fight them fair and square. Then, on the next full moon, Mangetsu and Kuon face off against JI GUAN LONG together according to a plan they devised before the battle, while Shingetsu draws Anna away from them. As shots are exchanged, Mangetsu finally figures out where JI GUAN LONG is
After getting a glimpse of Nene's past, Mangetsu realizes what it means to fight in GRANBELM, but at the same time finds herself with even more questions about it. Still, now she knows that every participant has her own reasons for fighting. Meanwhile, Anna attempts to steal the Fugo family's powerful magic stone, which leads Suishou to give up on her and leave the Fugo household.
Shingetsu learns that Anna's mother was attacked and hurries to visit her at the hospital. She then makes up her mind to find Anna, who has gone missing, and defeat her once and for all. When the full moon rises, the next GRANBELM battle begins, and Shingetsu is faced with a much more powerful Anna... thanks to the power of the Fugo family's magic stone.
Mangetsu and Kuon are overwhelmed by Suishou's unsettling aura as they fight her, barely managing to hold out until the end of the battle. Mangetsu awakens back at Shingetsu's house, then sets out to visit Nene after receiving a message from her. They discuss the events of last night's battle, but Nene has no recollection of Anna whatsoever. Perturbed, Mangetsu and Shingetsu hurry to the Fugo household, but what they find isn't what they expected...
While Nene is investigating abnormal magic signals, she realizes that Mangetsu lies at the center of the astronomically high readings. Meanwhile, as Kuon is thinking about her older sister Shisui, who has long been asleep due to a curse, she somehow receives a message from her. She heads toward the location indicated in it and finds none other than the one responsible for the curse: Suishou.
Mangetsu and Shingetsu have been expertly played by Suishou, leaving them unable to fight her effectively. Kuon tries to get them to leave the area and sets up a powerful attack aimed at Suishou, but a moment's inattention leaves her open to a vicious counterattack...
Mangetsu, who has become uncertain of her own existence, runs into Kuon's older sister, Shisui. As she talks with Shisui, Mangetsu feels moved to send a message to Shingetsu. Meanwhile, Shingetsu is convinced that Mangetsu is suffering because of the wish she made, and her guilt brings her to tears. As she's talking with Mangetsu, she becomes unable to bear it any longer and runs from the house...
Shingetsu heads into the final battle with Mangetsu's feelings held within her heart. Suishou is fighting to prove that no human is worthy of becoming the one true mage, while Shingetsu and Mangetsu are fighting to rid the world of magical power for good. As the battle intensifies, Shingetsu becomes caught within Suishou's grasp. Mangetsu tries desperately to save her, but Shingetsu takes the full force of the beam Suishou fires...
GRANBELM is finally down to two participants. Their mechs surpass their own limits, but eventually they can no longer withstand the impact sustained during the battle, and their functions begin to fail. When Shingetsu's situation grows dire, she's surprised by the appearance of Mangetsu, who was supposed to have disappeared...
