In a dark void known as the Lightless Realm, a group of children meet and bond, but are then separated and returned to their own dimension. Years later, these children reunite to fight "Admonitions," monsters that originate from the same void, using robots known as "Vess." However, their efforts only slow the ever-encroaching and expanding Lightless Realm from infecting their world. Their lives take a drastic turn when a new foe appears in the form of a "Corpse," whose song is rumored to kill any who hear it within nine days. The "Corpse Song" is heard by the group, and with this, they each begin to pull away and become caught up in their own personal problems—ultimately threatening the fate of humanity.

It was Meiji 38 (1905), the Russo-Japanese War that had begun since the previous year was getting hard. In this severe circumstance, they reorganized the Raimu Squad. The members of the Raimu Squad were the girls. They were just like other ordinary girls who went to schools and yearned for love, except that they had "Raimu." Inukai Tsuyoshiro was a young man who was newly chosen to be the leader of the squad. He was ambitious to fight for Japan. When the young man met the five girls, the new chapter of "Raimu" began.

As society rises from the ashes of war, cybernetically augmented arena fighters battle for fame and fortune... or die trying. It's the 19th century, and the world has entered the Era of Rebirth, recovering from the devastating flames of war. The sport of mechanical martial arts has galvanized the nations. Cybernetically augmented fighters turn their blood into steam and their bodies into brutal fighting—and killing—machines. Young Levius is one of those arena battlers, hell-bent on winning in order to simply survive.

The day before the ceremony marking the completion of the Yajima Stadium, Nils Nielsen invites Sei Iori, Meijin Kawaguchi, and others to an exhibition battle using the new large-scale battle system he has designed. Now that they are all reunited, Sei attempts to show off the Build Strike Cosmos that he has upgraded for the exhibition, but then he notices something missing... Meanwhile, somebody takes control of the stadium's systems, sealing Sei and the others inside. Before them appears a mysterious group, which has come to challenge them to Gunpla Battle.