The main plot of Getter Robo is the Getter Rays and the destruction that they induce. Getter Rays seek to evolve into the highest form possible, which is Getter Emperor. Getter Emperor is literally the nemesis of the universe due to the fact that it absorbs everything and will expand in size until it devourers the universe. When Getter rays were first discovered and harnessed into Getter Robo, enemies started to attack. Beings with the power of god exist to stop Getter rays because of the Getter rays, but Getter rays warp time and space to further its own ends.
Also, the gurren lagann spiral eyes was done by Getter and is one of the more memorable things.
In the last part of New Getter Robo, Ryouma and co discover the remains of the Getter soldiers. They are the Getter Robos left behind by those who fought the enemy of Getter. When Getter Robo discovers this, all of the Getter energy of the fallen soldiers flow into Getter Robo. Sounds really fucking familiar, doesn't it? This is just one example.