Super Robot Retsuden

Super Robot Retsuden is a spin-off manga made by Dynamic Productions featuring the company's most well known mecha: Mazinger Z, Steel Jeeg, Getter Robo (later Getter Robo G), Great Mazinger, and Grendizer. The events are non-canon to all of the series that are featured.

Plot
An alien lifeform named Varon the Annihilator appears on radar before giving a message to planet Earth that he will destroy the planet's civilization with his Meteor Beasts. The robots from the Photon Power Laboratory, Build Base, Saotome Institute, and Science Fortress Laboratory prepare for battle against the monsters occasionally helping each other out when they are backed against the wall.

Soon enough the robots face against Varon when given equipment to fight in space. With help from the Grendizer, they manage to destroy Varon's body, presumably at the cost of Jeeg. Unfortunately, his nucleus survived and went back to Earth to recover enough energy. With the Getter Robo damaged, and Musashi Tomoe injured the Getter team changed labs and gained the Getter Robo G. As Varon attacked from the ocean, Jeeg returned explaining his weaknesses to the Getter. As Hiroshi is repaired he joins the other robots on the moon to assist in using the convoy to fight against Varon. As Koji finds a signal on the moon itself rather than from space as anticipated, the Getter goes to investigate only for Varon to appear out of the ground. Varon proves to be very powerful, so much that even the convoy alone is not enough. As the robots combine their energies with the convoy, it transforms into the Hyper Robo Dynamic Saga. Even with this new weapon, Varon is still taking much of the machine's energy. Once in close proximity, the robot launches the Great armed with a Mazinger Blade onto Varon's core, destroying him in an explosion. Triumphant, the pilots proclaim that no matter what the threat their robots will protect the Earth.

Trivia

 * As Ken Ishikawa did most of the artwork, the Getter Robo and Getter Robo G received the most attention compared to the other robots.