The near future. Years after the huge "Gunpla"-plastic models based on the anime series Mobile Suit Gundam-boom of the 1980s. A second Gunpla boom is taking the world by storm, fueled by the revolutionary new ability to pilot your own Gunpla creations in "Gunpla Battles". These incredibly-popular Battles culminate in a yearly global tournament. Our hero Sei Iori, is a young Gunpla builder who dreams of someday making it to the world championships. An only child whose family owns a model shop, his talent is well-honed, but his weak piloting abilities have led to a series of first-round losses. Then, one day, he meets a boy named Reiji whose piloting skills are incredible. Together as a team, they take on the world! Sei, the Gunpla Builder. Reiji, the Gunpla Fighter. Sei builds, Reiji fights. Their dream is to make it to the Gunpla Battle World Championship... and win!

Seven years have passed since Sei Iori won the 7th Gunpla Battle Championship World Tournament. The Gunpla Battle Championship is now run by Yajima Trading. With new rules and new battle modes, the game's popularity has become even more widespread. However, the Seiho Academy that Sei Iori once attended has failed to catch onto the trend. The only member of the school's Gunpla Battle Club is its president, middle schooler Fumina Hoshino. Currently, there is no way the club can participate in the middle and high-school division of the upcoming All-Japan Gunpla Battle Championship, where players fight in teams of three. Then Fumina meets a new transfer student. He is Sekai Kamiki, a young martial artist who has been traveling with his master. Together with a young Gunpla builder named Yuuma Kousaka, they finally have three members for their team.