A strange radio transmission, originating from an unknown planet 3,000 light-years away from the solar system, races across the universe towards Earth. One night in May 2001, a laboratory receives this extraterrestrial signal and establishes contact. The message reveals that the Green Demon King from planet Cosmo is preparing to invade Earth. The professor, facing the emergency, convenes a conference of scientists, but his colleagues do not believe him. To gather more solid evidence, the professor embarks on Rambot with Godori and Taenggu to head to planet Cosmo. However, upon arrival, a robot appears before them, and Professor Kang is kidnapped by the Green Demon King. The Green Demon King kidnaps Professor Kang, the creator of Rambot, a major obstacle to his conquest of Earth, and subjects him to mental modification to make him his subordinate. He then uses Professor Kang to destroy Rambot's brain circuit. The Green Demon King attacks Earth, defeating the Alpha 1, Beta 2, and Gamma 3 units of the Earth Defense Force, and kidnaps Haeni, Professor Kang's daughter. Learning that Haeni possesses a spare brain capable of piloting Rambot, Godori and Taenggu go in search of her, infiltrate the Green Demon King's main battleship, and are captured. However, with Kanto's help, Godori escapes with Rambot's spare brain and manages to activate it. Godori, piloting Rambot, defeats Viking X, while Professor Kang, regaining his senses thanks to Kanto, escapes with Taengbo and Haeni. The Green Demon King counterattacks aboard the Black Mask but is destroyed by Rambot. The remaining green inhabitants, guided by Kanto, abandon their weapons and return to Cosmo.