Duke Fleed, prince of the annihilated planet Fleed, commandeers the experimental Yaban war machine Gattaiger after his world's destruction. Fleeing to Earth, he adopts the human identity Daisuke Umon and conceals both his origins and the Gattaiger within the Makiba family ranch. He deliberately abandons warfare for peaceful coexistence on his adopted world.

The relentless Yaban forces under General Blaki eventually trace him to Earth, unleashing devastating saucer assaults on human cities. Fleed reactivates the Gattaiger to defend Earth, driven by protective instincts toward his new home and vengeance against his planet's destroyers. The Gattaiger deploys advanced weaponry: faster-than-light flight, the rotating Spider Spin attack, projectile Needle Shower missiles, and concentrated energy beams designated Space Thunder and Thunder Focus.

Complicating the conflict, Yaban heir Telonna—Fleed’s former betrothed—arrives on Earth. Torn between imperial duty and personal longing, she advocates for Fleed’s return to rebuild Fleed under peace, even as she oversees military operations against humanity. This duality strains both, forcing them to reconcile past affection with present loyalties.

During the climactic battle, Telonna intervenes to halt the bloodshed but suffers fatal wounds. Her death extinguishes Fleed’s hopes for reconciliation or restoration of their homeworld. He severs ties with Earth, departing aboard the Gattaiger as a solitary exile. His trajectory shifts from prince to refugee, reluctant warrior to cosmic drifter—marked by the tragedies of Fleed’s destruction and Telonna’s loss. The narrative underscores his enduring burdens: displacement, warfare’s repercussions, and the collision of duty with love.