Gundam Build Fighters: GM no Gyakushuu

On the eve of Yajima Stadium's completion ceremony, Nils Nielsen gathers Sei Iori, Meijin Kawaguchi, and their old friends to test a new large-scale Gunpla Battle system. Sei is eager to unveil his upgraded Build Strike Cosmos, but the reunion is interrupted when someone seizes control of the stadium and locks everyone inside.

A mysterious team then challenges the trapped fighters to Gunpla Battle, forcing old rivals to fight side by side. With the arena itself under hostile control and unfinished business from the past resurfacing, Sei and his friends must prove how much their skills and bonds have grown since the world tournament.