"Getter Robo G" is a sequel to the original "Getter Robo" series, continuing the saga of humanity's struggle against the Dinosaur Empire and other extraterrestrial threats. The story picks up after the defeat of the Dinosaur Empire, with the Getter Team disbanded and the original pilots—Ryoma Nagare, Hayato Jin, and Musashi Tomoe—going their separate ways. However, peace is short-lived as a new enemy, the Hyakki Empire, emerges. This advanced alien civilization, led by Emperor Gore, seeks to conquer Earth and eliminate humanity.

To combat this new threat, Professor Saotome, the creator of the Getter Robo machines, develops an upgraded version of the original Getter Robo, known as Getter Robo G. The new mecha is more powerful and versatile, capable of transforming into three distinct configurations: Getter-1 (air combat), Getter-2 (land combat), and Getter-3 (sea combat). Ryoma and Hayato return as pilots, but Musashi is replaced by Benkei Kuruma, a hot-headed martial artist with a strong sense of justice. Together, they form the new Getter Team, tasked with defending Earth from the Hyakki Empire's relentless assaults.

The Hyakki Empire deploys a variety of monstrous biomechanical creatures and advanced war machines to achieve their goals. The Getter Team faces numerous battles, each requiring strategic use of Getter Robo G's transformations and weapons to overcome the enemy's unique abilities. The series explores themes of teamwork, sacrifice, and the resilience of humanity in the face of overwhelming odds. As the conflict escalates, the Getter Team uncovers the true nature of the Hyakki Empire and its connection to ancient Earth history, adding layers of intrigue to the narrative.

Throughout the series, the characters grapple with personal challenges and growth. Ryoma remains a skilled and determined pilot, but his brash demeanor often puts him at odds with others. Hayato, the more level-headed and strategic member, serves as a counterbalance to Ryoma's impulsiveness. Benkei, despite his initial lack of experience, proves his worth through sheer determination and loyalty. The dynamic between the pilots is central to the story, highlighting their evolving relationships and shared commitment to protecting humanity.

The narrative builds toward a climactic confrontation with Emperor Gore and the Hyakki Empire's ultimate weapon, culminating in a high-stakes battle that tests the limits of the Getter Team and their mecha. The series concludes with a resolution to the immediate threat, but leaves room for future conflicts, reflecting the ongoing nature of humanity's struggle against extraterrestrial forces. "Getter Robo G" is a foundational entry in the mecha genre, blending intense action, character development, and a compelling storyline that has influenced subsequent works in the genre.