"Turn A Gundam" is set in a distant future where humanity has regressed to a pre-industrial society following a catastrophic event known as the "Moonlight Butterfly." The story unfolds on Earth, where the technologically advanced Moonrace, a civilization that fled to the moon centuries ago, returns to reclaim their ancestral home. The Earth's inhabitants, living in a largely agrarian society, are unprepared for the Moonrace's advanced technology and military might.

The protagonist, Loran Cehack, is a young Moonrace scout sent to Earth to assess its readiness for the Moonrace's return. Disguised as a humble Earthnoid, Loran integrates into the Heim family, working as a mechanic. When the Moonrace begins their invasion, Loran discovers the Turn A Gundam, a powerful and enigmatic mobile suit buried in the mountains. Reluctantly, he pilots the Turn A to protect the Earthnoids, despite his loyalty to the Moonrace.

The conflict escalates as the Moonrace, led by figures like Gym Ghingnham and Dianna Soreil, clash with Earth's resistance forces, including the militia led by Sochie Heim and Guin Lineford. Loran's dual identity creates tension, as he struggles to mediate between the two factions while uncovering the truth about the Turn A Gundam and humanity's forgotten history. The Turn A is revealed to be a relic of a bygone era, capable of immense destruction, and its existence becomes a focal point for both sides.

As the war progresses, alliances shift, and characters grapple with their motivations. Dianna Soreil, the Moonrace queen, seeks a peaceful resolution, while Gym Ghingnham desires to dominate Earth. Loran's actions and the Turn A's power force both sides to confront the consequences of their actions and the cyclical nature of war. The narrative explores themes of reconciliation, the burden of history, and the potential for coexistence between disparate civilizations.

The series concludes with a climactic battle that forces both Earthnoids and Moonrace to reconsider their futures. The Turn A Gundam's true purpose is revealed, and Loran's efforts to bridge the divide between the two factions culminate in a fragile but hopeful resolution. The story emphasizes the importance of understanding and cooperation, leaving the fate of humanity's coexistence open-ended but optimistic.