Sei Iori aims to participate in the Gunpla Battle Championship World Tournament, but despite his skill at building models, he can't control them in battle. Then Sazaki challenges him to a Gunpla battle in order to get his hands on Sei's high-performance Gunpla. Sei accepts the battle, thinking that if he can't even beat Sazaki, then he'd be defeated instantly if he made it to the championship. But Sei is backed into a corner by Sazaki's overbearing fighting style until the appearance of Reiji, a mysterious boy who he met earlier that day. Even though Reiji has never used Gunpla before, he…