「オトナっていうのはね、子供の笑顔が大好きなんだ」 大也の先生・内藤から届け屋に入電。 依頼はキャンプをする孫・千二に花火を届けてほしいというもの。 しかし千二が遊ぶ山中に苦魔獣が出現。 偶然キャンプをしていた先斗が駆けつけるも隙を突かれダメージを負ってしまう。 大也も合流するが、激闘の末に苦魔獣は巨大化。 更にはキャノンボーグの作戦でネジレッタたちも巨大な姿を手に入れてしまう。 多くの敵が脅威となるピンチの中、 ブンドリオや射士郎たちが作りあげた新たなブンブンカーが到着。 その車は一見消防車のようでもあり、赤き獅子の姿も持っていた…。 燃える山に 獅子、吼える──。
``Adults love to see children's smiles.'' Daiya's teacher, Naito, calls the delivery shop. The request is to deliver fireworks to his grandson, Senji, who is camping. However, a beast appears in the mountains where Senji is playing. Ponto, who happened to be camping, rushes in, but is caught in the crossfire and sustains damage. Daiya joins them, but after a fierce battle, the beast becomes gigantic. Furthermore, due to Cannonborg's strategy, Nejiretta and his friends also acquire gigantic forms. In a pinch where many enemies are a threat, a new Bunbunker created by Bundrio and Yasushirou arrives. At first glance, the car looked like a fire engine, and it also had the appearance of a red lion... A lion roars on a burning mountain.
