A giant robotic version of the toy Vulcan swoops down from the sky and kidnaps the human genius Kiyomaro Takamine. The culprit is a mysterious figure named Dr. M2, a magic scientist from the future who wants Kiyomaro to become his partner. While Kiyomaro wakes up trapped in Dr. M2s laboratory, the young demon Gash Bell is left behind, confused and upset. Gash soon encounters the same robot that took his friend, and believing it to be a new friend made by Kiyomaro, he names it Volcan 300 Generation 4. Through a glitch in the robots system, Generation 4 agrees to help Gash rescue his partner.

The story takes place in and around Mochinoki Town, the familiar setting of the series, but soon expands into a fantastical conflict involving a time warp. Dr. M2, who commands an army of similar robots called Death Mechanics, reveals that he needs Kiyomaros genius to perfect his mechanical army. He attempts to persuade the boy to abandon his friends and come to the future. However, Gash, along with his demon friends Tio, Kanchome, and Umagon, interfere with the plan. As they infiltrate Dr. M2s floating castle, a fierce battle breaks out. During the fight, Kiyomaro comes to a heartfelt realization about the value of friendship, understanding that the simple toy Vulcan he carelessly made for Gash is more precious than any advanced machine.

The climax shifts from a simple rescue to a city-wide disaster when the time warp Dr. M2 used becomes unstable and threatens to consume everything. The group must work together to seal the rift. In a desperate plan, Generation 4 sacrifices itself by attaching to the core reactor. Gash and Kiyomaro combine their strength to unleash a powerful Baou Zakeruga spell, which successfully reactivates the reactor and closes the time-space hole, saving the city from annihilation. In the end, Kiyomaro promises Gash that while they may not see Generation 4 again, he will build him an even better Vulcan, symbolizing the unbreakable bond of friendship they share.