UFO Robot Grendizer was developed to be a sequel to Great Mazinger after the initial concepts of a sequel were rejected by Toei. With the appearance of Uchu Enban Daisenso at the March 1975 Toei Manga Festival a remake was pitched using elements from Mazinger. To breathe new light into the series, the villains of the series were decided to be aliens, a trend that followed in other Toei mecha series while also making Duke a more Blue Blood character with his suit being based on a knight including more romantic elements with its characters and sense of adventure.
Go Nagai however stated in an interview that the anime series was considered a fun side project and does not consider Grendizer to be part of the series timeline. This is mainly because he had a lot of disagreements with Toei and Shingo Araki.
Grendizer focuses on protagonist Duke Fleed, the crown prince of the now ruined Planet Fleed and the Super Robot Grendizer. When the Vegan Empire attack the peaceful Planet Fleed to expand their military rule as a result of the radiation emanating from their former home planet; they opt to steal the Grendizer for their own exploits. The robot was stolen by the surviving prince and using a Spazer to escape came to Earth. Duke is adopted by scientist Dr. Genzo Umon, as his own son and is renamed Daisuke Umon. Working on a ranch with his friend Hikaru Makiba and her family, they get a visit from space travel student Koji Kabuto who then goes to meet a swarm of UFOs that have been appearing around Japan, unaware that UFOs are the Vegan Empire. Duke saves Koji with Grendizer and begins a long battle against the Vegan Empire with Koji, Hikaru, and Duke's lost sister Maria Fleed joining him.
Grendizer was the second longest running animated Mazinger series in Japan, having 74 episodes. It also was a major success in France and Italy, there known as respectively Goldorak and Goldrake. It was also one of the first anime programs to be a major success in Europe and Canada, where it continues to have a strong following. It also extremely popular in the Arab World. Several people however complained about its connection to the other Mazinger series in the franchise, especially with Koji's status as a mere sidekick as well as the lack of the other mechs.