The Vega homeworld is becoming 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 neighbours such as Fleed, a highly advanced but peaceful world. In a tragically ironic twist, the invaders' blitzkrieg turns against them: the once verdant, idyllic Fleed is turned into a radioactive wasteland. Too late, the only known survivor of the royal family, Prince Duke Fleed, 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.

Fleeing Vegan space by flying at faster than light speed, Duke enters the Solar system and switches course to Earth, making a rough landing in Japan, on the slopes of Mount Fuji ...

Source: Wikipedia

Note: Episode 49 was also shown at the Toei Manga Festival event on 19.12.1976 under the changed title Akai Yuuhi no Taiketsu (赤い夕陽の対決).

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Prequel: Great Mazinger
Same characters: Grendizer, Getter Robo G, Great Mazinger: Kessen! Daikaijuu!
Side story: UFO Robo Grendizer Tai Great Mazinger
Alternative setting: Uchuu Enban Dai-Sensou
Other: Robot Girls Z
Alternative version: Grendizer Giga
Alternative version: Grendizer U