"Itsuka no Main: Kaminari Shōnen Tenta Sanjō" centers on Tenta, a young boy with a unique ability to summon and control lightning. Set in a rural Japanese town, the story begins with Tenta discovering his powers during a thunderstorm, which he initially struggles to understand and control. His life takes a dramatic turn when he encounters a mysterious girl named Main, who claims to have traveled from the future. Main reveals that Tenta’s powers are tied to a catastrophic event that will occur decades later, and she seeks his help to prevent it.

As the narrative unfolds, Tenta learns that his abilities are part of a larger phenomenon involving other individuals with elemental powers. These individuals, known as "Kaminari Shōnen," are scattered across different timelines and dimensions, each wielding control over a specific natural element. Main explains that the impending disaster is linked to the misuse of these powers by a rogue faction seeking to manipulate time and space for their own gain. Tenta, initially reluctant to embrace his role, gradually accepts the responsibility of protecting the timeline after witnessing the consequences of inaction.

The story explores themes of destiny, sacrifice, and the ethical implications of wielding immense power. Tenta’s journey is marked by his growth from a hesitant, ordinary boy to a determined protector of the timeline. Alongside Main, he forms alliances with other Kaminari Shōnen, each with their own motivations and struggles. The group faces numerous challenges, including battles with the rogue faction, internal conflicts, and the moral dilemmas of altering the future.

The narrative is structured around a series of interconnected arcs, each delving deeper into the lore of the Kaminari Shōnen and the complexities of time travel. Key characters include Main, whose enigmatic past and connection to Tenta are gradually revealed, and the antagonists, who are driven by a mix of ambition and desperation. The story balances action-packed sequences with moments of introspection, highlighting the emotional and psychological toll of Tenta’s journey.

By the conclusion, Tenta and his allies confront the rogue faction in a climactic battle that tests their resolve and unity. The resolution ties together the various narrative threads, offering a satisfying conclusion while leaving room for interpretation regarding the broader implications of their actions on the timeline. The series stands out for its intricate plot, well-developed characters, and exploration of timeless themes, making it a compelling addition to the genre.