Deep within the Earth, dinosaurs thought to be extinct seek to take Earth back from the humans. Dr. Saotome recruits three pilots: Ryoma Nagare the karate master, Hayato Jin the rebel terrorist, and Musashi Tomoe the former captain of a judo team. Together they pilot Saotome's ultimate creation, the Getter Robo, to stop the Dinosaur Empire from taking over Earth.
The Saotome Institute has been studying the Getter Ray Energy that has showered the Earth since the dawn of time for a new energy resource. However, the Dinosaur Empire plans to retake the surface by taking out the institute and its research as Getter Rays are toxic to them. To protect the institute and humanity, Dr. Saotome creates the Getter Robo, a robot that can utilize the Getter Ray energy to fight the empire. However the jet component's performance is very strenuous and requires pilots with exceptional abilities to control them. Dr. Saotome sends out scouts that push candidates to their limits to find the worthy pilots. The first was Ryoma Nagare, a martial artist that utilizes a pragmatic karate style inherited from his late father. The next was a homicidal student activist who dominated his entire school, Hayato Jin. While Dr. Saotome intended to be the third pilot himself, a Judo fighter named Musashi Tomoe proves his worth despite not possessing the minimal requirements.
Near the end of the manga, Ryoma is captured by another villainous group called the Hyakki Empire. He was tortured by them to forget about how to control the Getter Robo and be their servant which hinders his performance. The Dinosaur Empire attempts to attack the Saotome Institute with an army of incomplete but capable Mechasaurus. Dr. Saotome is working on a new and stronger Getter Robo as the base Getter doesn't have enough power to fight back. Musashi is aware of this and is told by Dr. Saotome that there isn't enough time to finish. So Musashi comes up with a plan that Dr. Saotome agrees to and doesn't tell the others about. In the Getter Robo, Musashi controls all three jets and modes before he makes the Getter Core self-destruct which takes out the Mechasaurus army but also takes Musashi's life. As the others mourn for Musashi, Ryoma regains the ability to control the Getter Robo and vows to finish the Dinosaur Empire.
The empire's ruler, Emperor Gore goes out to the ruins of the Saotome Institute and falls for a trap with the Getter Robo G complete. Gore attempts to flee into the ocean where a fleet of Mechasaurus are waiting. However, before anything could be done, the Hyakki Empire wipes out the navy and Emperor Gore was killed by a Hyakki Beast as he was about to fight off the new Getter. With the Hyakki Empire now out and about, Ryoma and Hayato prepare for them.