"Grendizer U" is a continuation of the classic mecha anime series "UFO Robo Grendizer," originally created by Go Nagai. The narrative centers on Duke Fleed, a prince from the planet Fleed who seeks refuge on Earth after his homeworld is destroyed by the malevolent Vega Empire. On Earth, Duke assumes the identity of Daisuke Umon and pilots the powerful robot Grendizer to protect humanity from the Vega Empire's relentless invasions.

The Vega Empire, led by the ruthless King Vega, seeks to conquer Earth and exploit its resources. Duke, alongside his allies Dr. Umon, his daughter Hikaru, and fellow pilot Koji Kabuto, forms a resistance to thwart the Empire's plans. The series explores themes of loyalty, sacrifice, and the struggle for survival as Duke balances his dual identity and the weight of his responsibilities.

Key characters include Duke Fleed, whose internal conflict and determination drive the narrative; Hikaru Umon, who provides emotional support and scientific expertise; and Koji Kabuto, a skilled pilot who aids in the fight against the Vega forces. The antagonists, such as King Vega and his commanders, are portrayed as formidable adversaries with their own motivations and strategies.

The plot unfolds through a series of battles between Grendizer and the Vega Empire's advanced weaponry, with each encounter escalating the stakes. The series maintains a balance between action-packed mecha combat and character-driven drama, highlighting the personal struggles and growth of its protagonists. As the conflict intensifies, alliances are tested, and the true extent of the Vega Empire's ambitions is revealed, leading to a climactic confrontation that determines the fate of Earth and its defenders.