Duke Freed, Prince of the doomed planet Fleed, escapes its annihilation by the Yarban Empire, commandeering their experimental weapon, the Gattaiger. He crash-lands near Mount Fuji on Earth, where scientist Dr. Genzo Umon finds and saves him. Adopting the alias Daisuke Umon, Duke hides his alien origins, embracing a peaceful life as a rancher on his adoptive world. This existence offers a fragile respite from his traumatic past, though memories of his lost planet and people linger.

Five years later, Yarban forces locate Duke on Earth, invading with fleets of saucer craft. Facing his enemies once more, Duke abandons tranquility and reactivates the concealed Gattaiger to defend Earth. The conflict deepens with the arrival of Princess Teronna (also called Telonna), heir to the Yarban throne and Duke's former fiancée. Torn between affection for Duke and imperial duty, Teronna seeks reconciliation, urging him to return with her and rebuild Fleed. Duke, distrustful of her loyalty to the empire that destroyed his home, rejects her plea.

The decisive clash against Yarban commander Blaki culminates in Teronna sacrificing her life to shield Duke from an attack. Her death extinguishes any hope of reconciliation. With the immediate Yarban threat ended and Teronna gone, Duke honors his vow to depart Earth. Piloting the Gattaiger, he leaves behind the life of Daisuke Umon and vanishes toward an undisclosed future.