A university student named Goro Watari possesses latent psychic abilities, a power that draws him into a secret war against the Neo-Human Empire, a sinister organization of mutants led by the maniacal Emperor Banba. After demonstrating his powers by saving two children from the Empire's forces, Goro is recruited by the Youth League, a covert group of psychics dedicated to protecting humanity. The League's leader awakens Goro's full potential, enabling him to become a powerful mutant warrior.

Goro’s transformation is unique, occurring in two distinct stages. When he crosses his arms and shouts Goriki Shorai (Summon Mighty Power), he first becomes Sanagiman, a brown, armored, pupa-like form. In this state, he can absorb kinetic energy from enemy attacks. Once he has stored enough energy, he shouts Choriki Shorai (Summon Super Power), causing his Sanagiman shell to burst open and reveal his final form, Inazuman. This moth-like hero is characterized by large oval eyes, lightning-shaped antennae, a yellow scarf that can transform into various weapons such as a chain or whip, and a full array of psychic abilities including telekinesis and energy projection. Goro’s signature fighting cry is Chest, a phrase from his native Kyushu.

The series takes place in 1970s Japan, where the Neo-Human Empire, operating from a dark, misty cave, seeks to eradicate ordinary humans (whom they call the "old race") and establish a world for mutants. Emperor Banba, a powerful psychic who can control super-high heat flames, deploys a weekly corps of grotesque Mutant Creatures and Phantom Soldiers to carry out his schemes, which range from industrial sabotage to biological warfare and mass kidnappings. Goro is often supported by his best friend, the comedic Gosaku Marume, and his allies in the Youth League, including members Satoko and Katsumi Oki. He also has a telepathic bond with a sentient, flying supercar named Raijingo, which is equipped with jaws and hidden weaponry to aid him in battle.

The story is split into two distinct narrative arcs. In the first 25-episode arc, Goro repeatedly clashes with Emperor Banba's forces. A notable subplot involves the return of Goro’s long-lost mother, Shinobu Watari, who has been transformed into the mutant Rose Bambara and brainwashed by the Empire. She ultimately sacrifices herself to save her son. As the series progresses, Emperor Banba’s power wanes due to Inazuman’s constant victories and internal strife. In the finale of the first arc, a new, more threatening force called the Despar Army arrives, led by Fuhrer Geisel. They forcibly absorb the remnants of the Neo-Human Empire. A desperate Emperor Banba initiates a final plan to sink the Japanese archipelago but is ultimately defeated by Inazuman. However, Goro's victory is short-lived, as the emergence of this new enemy immediately sets the stage for the next chapter, the second 23-episode arc titled Inazuman Flash, which features a darker tone and a new villain.