Alien invaders take over Great Mazinger who is at the entrance of a museum next to Mazinger Z as symbols of peace. Grendizer fights the evil invaders and Great Mazinger is recovered. 
When the Vegan empire dispatches a new general to oversee the takeover of Earth, he quickly manages to capture Koji Kabuto and, learning of Great Mazinger through interrogation, quickly commandeers it from a museum where it was kept as a symbol of peace. 
It is now up to Duke Fleed aboard Grendizer to stop humanity's former defender from becoming its destroyer.
 
(Source: Anime News Network)

A short manga version of the UFO Robo Grendizer anime. Released along the anime series, UFO Robot Grendizer follows the adventures of Duke Fleed, the Prince of Planet Fleed, whom disguised as Daisuke, a farm worker, fights against the forces that destroyed his planet, the Vegan Empire, also the creators of Grendizer that Duke stole to fight them. While not originaly considered part of the Mazinger series, Koji Kabuto and the Mazinger mechas appear in this series, making it sort of a sequel. Grendizer introduced the concept of a transformable Robot and a space Mecha, concepts that were later taken into other mecha series. It was extremely popular in Japan, Canada, France, Italy and the Arab World and has a strong following in those countries even today. (mangaupdates.com)

A manga comic book and anime television series by manga artist Go Nagai, made as a direct continuation of the successful Mazinger Z series. It was aired on Japan in 1974, immediately following the end of the first Mazinger series.

From the island of Bardos in the Aegean Sea comes a new threat to humanity in the form of Doctor Hell and his army of mechanical beasts. However, a new hero stands in his way: Mazinger Z, a giant robot built out of the indestructible metal chogokin and equipped with an arsenal of advanced weaponry. Piloting Mazinger Z is hotheaded teenager Kouji Kabuto, who strives to avenge the death of his grandfather and protect Japan from Doctor Hell's forces.
 
(Source: Anime News Network)

Duke Fleed is a survivor of the Fleed Planet who is escaped to the Earth, onboard a mysterious "spaceship", where is raised by Dr. Umon as his adoptive son, and known as Daisuke Umon. Years after his arrival, he's faced with the threat of King Vega and his army, who want to conquer the Earth. With his friends Koji and Hikaru (and later his younger sister Maria Grace), Duke decides to fight back using his best weapon - the almighty Grendizer.

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