A man who fell into the desert has no memory. He is named Daisuke and hits it off with Koji Kabuto, who picked him up. As unidentified forces attack the Earth, a giant robot called Grendizer appears.

Duke defeats the Vega forces, but loses his senses and flies away eventually landing on Ogami Island. Koji follows him and just as he meets the island's shrine maiden Hikaru, his arch enemy Cassado attacks Earth.

Duke's anger burns up in the despicable cassado. He makes Grendizer run wild, but he remembers his promise to Koji and stops. Duke regrets that he got on again and cheers of gratitude.

Reunion with childhood friend Naida. It brings a brief moment of peace to Duke. However, she approached Duke as an enemy of her brother Sirius. The blade of love and hate is approaching.

Duke's sister, Maria, came to earth after her brother. However, Maria looks down on the unexplored planet Earth with girlish arrogance. Meanwhile, the Vega army will attack again.

Teronna, a female knight who can be said to be Duke's best friend, landed on Earth. Unlike the previous Vega army, Teronna offers a fair game, but she condemns Duke as a planet sinner.

Investigation of super-ancient ruins was underway on Aijin Island. A new weapon of the Vega army is released in the nearby sea. Grendizer is dragged into an unexpected battlefield and has a hard time. At that time, a mysterious shadow approaches.

Duke receives a message from the flying beast and reunites with Rubina, who has come down to Earth. Duke begins to tell the truth about the tragedy on Planet Fleed.

The Photon Power Research Institute is finally directly attacked by the Vega Star Alliance. It is invaded by a melee combat unit led by Blacky, and the place becomes a battlefield. Duke, who was imprisoned on suspicion of being a spy, is saved by Umon.