Aliens don't just attack Earth, they steal humanity's greatest weapon right out of a museum and turn it into a tool of conquest. Duke Fleed has to fight the machine that was supposed to protect everyone, the one thing people were supposed to trust. This is what happens when your symbols of peace become your enemy's ammunition.

Duke Fleed pilots Grendizer against the invading Vegan empire. When the Vegans send a new general to oversee their conquest of Earth, he captures Koji Kabuto and learns through interrogation about Great Mazinger, which stands at a museum entrance beside Mazinger Z as a symbol of peace. The invaders seize the robot and turn humanity's former defender into a threat. Now Duke Fleed and Grendizer must stop the stolen machine. A 1976 mecha and science fiction film, it is Grendizer's first appearance on the big screen.