Uchu Enban Daisenso

Uchu Enban Daisenso (宇宙円盤大戦争 Uchū Enban Daisensō), translated and also known as Battlefield of The Space Saucers and The Great Battle of the Flying Saucers, is a Japanese animated short film created by Go Nagai. It served as a prototype for Grendizer with similar characters and mecha.

Story
Duke Fleed, the prince of the destroyed Planet Fleed escapes to Earth after stealing an experimental robot from the Yaban army known as the UFO Robo Gattaiger. There he is adopted by Genzo Umon and is given the name Daisuke Umon. However, the Yaban army attack Earth and Duke uses the Gattaiger to fight back. During the carnage, Duke encounters his fiance, Telonna but she is killed in the final battle against Blaki. With her death, Duke decides to leave Earth.

Adaptation to Grendizer
Using materials from Daisenso, Nagai and Dynamic Productions created Grendizer using some of the characters including Duke, the Makiba family, and even Blaki. Some changes were made to make it more original. The most obvious are the changes in design and the addition of characters such as Koji Kabuto and Maria Fleed.