The Vega homeworld has become unstable due to the exploiting of Vegatron, a powerful radioactive ore. Seeking to expand his militaristic empire and find a substitute planet to settle upon, the ruthless King Vega unleashes his armies—composed of flying saucers and giant robotic monsters—and turns first against neighbors such as Fleed, a highly advanced but peaceful world. The once verdant, idyllic Fleed is turned into a radioactive wasteland. Too late, the only known survivor of the royal family, the Crown Prince Duke Fleed (Actarus, prince d'Euphor in french), manages to steal the Grendizer, the robotic embodiment of the Fleedian God of War, from the Vegan invaders who plan to use it to spearhead their invasion fleet. Grendizer is a giant monster robot that interfaces with Spacer (Spaizer), a flying saucer that enables the robot to fly. Fleeing Vegan space by flying at faster than light speed, the Duke enters our solar system and switches course to Earth, making a rough landing in Japan, on the slopes of Mount Fuji. He is befriended by Doctor Umon, a noted scientist who oversees a research laboratory called the Space Science Lab near a small ranch. The kindly Umon takes in the young humanoid alien as his son, under the assumed name of Daisuke, and assists him in hiding Grendizer. Taking the name Daisuke Umon, Duke Fleed works at the ranch run by Danbei Makiba (based on Abashiri Daemon of Go Nagai's manga Abashiri Ikka). Roughly two years later, Koji Kabuto, after studying abroad, returns to Japan in a flying saucer he personally designed and built, called the TFO. He heads to the Space Science Lab after hearing of multiple sightings of "flying saucers". He plans to contact the aliens if possible and make peace with them. Daisuke, however, scoffs at the notion and fears that these aliens, the Vegans, led by generals Blaki and Gandal, are preparing to attack Earth. Koji ignores his warnings and flies out to meet the incoming saucers, only to discover the horrible truth. In order to save Koji and protect his adoptive homeworld from destruction, Daisuke is forced to return to his true identity as Duke Fleed. He unearths Grendizer from its hiding place under the lab and sets off to fight his enemies. The Vegans establish a base on the far side of the Moon and start to attack Earth from there. Koji discovers Duke Fleed's true identity and their bitter rivalry soon turns to friendship. The daughter of Danbei Makiba, Hikaru, also discovers Daisuke's secret and becomes a pilot in order to assist him despite his objections. Later on, it is revealed that there were two more survivors from planet Fleed: Duke's younger sister Maria Grace Fleed and a man who had rescued her and fled to Earth, raising her under the guise of her grandfather. Caught in a crossfire between Grendizer and a Vegan beast, he reveals to Maria that she is the last survivor of the royal family of Fleed (under the belief that Duke was killed) before dying from his wounds. Maria swears revenge on Grendizer and its pilot. She tries to ambush Duke, Koji and Hikaru at the Space Science Lab, but the fight is short. Maria's attacks bring Duke's necklace (which is the same as the one she wore) into view and the truth is revealed. The lost siblings are reunited at last and Maria becomes the last addition to the team.