On a future Earth shaped by rapid technological growth, the Ionoid Senshi Machine Robo are super-robots built to combat the disasters and accidents that plague high-tech society. Powered by a mysterious energy source known as Ionoid energy, these machines are equipped with a Romtron molecular composition conversion device that allows them to freely change size up to six hundred times their normal scale, enabling them to adapt to any crisis.

The central conflict pits the Machine Robo against the Demon, a colossal space monster that once wiped out the dinosaurs and now returns to threaten the planet. Standing alongside the robotic defenders is a young boy named Kiyoshi, who becomes a key figure in the struggle. The story unfolds as the Machine Robo and Kiyoshi face the Demon's assaults, drawing on the robots' transforming abilities and the boy's courage to protect Earth from a second extinction-level catastrophe. The narrative weaves together themes of cooperation between humans and machines, the legacy of ancient disasters, and the relentless advance of cosmic evil.