Takeshi is a 16-year-old orphan living at the Blue Sky orphanage who recently started an American football team. During one game, an aerial disturbance shatters the event, revealing his true origin: he is the son of Apolon's ruler. To save him from execution by the usurper General Dazaan, his father's retainer, Labi, sent infant Takeshi to Earth. This heritage implants innate energy-based powers called "Key Energy" within his chest. These abilities let him remotely command UFO saucers launching from an ocean base and initiate a solar-powered "body fusion" process. This process exponentially increases his physical mass to giant size, enabling him to integrate with and pilot the combined Dai Apolon robot as cybernetic armor. Takeshi's orphanage friends, Miki, Matsuo, and Goro, join him as pilots. They operate three distinct robotic components: Header (Head), Trangur (Trunk), and Legger (Leg), which merge to form the unified Dai Apolon entity. The team triggers an instantaneous transformation into specialized uniforms by shouting "U! F! O!". Takeshi defends Earth against threats orchestrated by General Dazaan while reconciling his human upbringing with his extraterrestrial royal lineage. His journey centers on leadership, the responsibility of his inherited powers, and protecting his adoptive home alongside his closest companions.