"Mobile Fighter G Gundam" is set in a future where Earth and its colonies have established a unique system to resolve conflicts: the Gundam Fight, a tournament held every four years. Each nation sends a representative piloting a Gundam, a humanoid mecha, to compete in battles across Earth. The winner earns their nation the right to govern the colonies until the next tournament. The story centers on Domon Kasshu, the representative of Neo Japan, who enters the 13th Gundam Fight with a dual mission: to win the tournament and to locate his older brother, Kyoji Kasshu, who has been accused of stealing the Devil Gundam, a highly dangerous and autonomous weapon.

Domon is a skilled but hot-headed fighter, driven by a sense of duty and a desire to clear his family's name. Accompanied by his mechanic and childhood friend, Rain Mikamura, Domon travels the world, encountering other Gundam Fighters and uncovering the truth behind the Devil Gundam. This mysterious machine possesses the ability to self-replicate and evolve, posing a catastrophic threat to humanity. As the tournament progresses, Domon learns that the Devil Gundam is not merely a weapon but a creation tied to his family's past, with Kyoji having been manipulated into its development.

Throughout the series, Domon faces a variety of opponents, each representing their nation's unique culture and fighting style. Notable rivals include Master Asia, a legendary Gundam Fighter and Domon's former mentor, who initially opposes him but later reveals a deeper connection to the Devil Gundam's resurgence. Another key figure is Schwarz Bruder, a mysterious masked fighter who aids Domon in his quest while hiding his own identity. The narrative explores themes of loyalty, sacrifice, and the consequences of unchecked technological advancement, as Domon and his allies work to prevent the Devil Gundam from achieving its destructive potential.

The climax of the series sees Domon confronting the Devil Gundam in a final battle, where he must reconcile his personal vendetta with the greater responsibility of protecting humanity. The resolution ties together the threads of Domon's journey, his relationship with Rain, and the fate of the Gundam Fight system itself. "Mobile Fighter G Gundam" combines intense mecha battles with a character-driven narrative, offering a distinct take on the Gundam franchise by blending martial arts, global exploration, and a focus on personal growth.