Three years after the Gundam Build Fighters TV series ended, Sei Iori and his friends reunite in a one-episode ONA. On the day before Yajima Stadium's completion ceremony, Nils Nielsen invites Sei, Meijin Kawaguchi and others to an exhibition match using a new large-scale battle system he designed. Sei hopes to show off his upgraded Build Strike Cosmos, but he realizes something is missing. Then someone seizes the stadium's systems and seals everyone inside, and a mysterious group appears to challenge them to Gunpla Battle. The result is mecha action built around model battles.