"Turn A Gundam" is set in a distant future where humanity has regressed technologically and socially, living in a world reminiscent of the early 20th century. The story unfolds on Earth, which is under the control of the Moonrace, a group of humans who fled to the Moon centuries earlier to escape a catastrophic war. The Moonrace returns to Earth to reclaim their ancestral home, sparking tensions with the Earth's inhabitants. The narrative centers on Loran Cehack, a young Moonrace scout sent to Earth to assess its readiness for colonization. Disguised as a Earthnoid, Loran integrates into the Hill family and becomes a close friend of their daughter, Sochie Heim.

The discovery of a mysterious mobile suit buried in a mountain, known as the Turn A Gundam, becomes a pivotal moment. Loran accidentally activates the Turn A, revealing its immense power and advanced technology. As conflict escalates between the Moonrace and Earthnoids, Loran is torn between his loyalty to his people and his growing attachment to the Earth and its inhabitants. The Turn A Gundam, a relic of a forgotten era, becomes a symbol of both destruction and hope, as its capabilities far surpass anything either side possesses.

Key characters include Dianna Sorel, the queen of the Moonrace, who seeks a peaceful resolution to the conflict, and her aggressive counterpart, Gym Ghingnham, who advocates for forceful colonization. Sochie Heim, driven by personal loss and a desire for revenge, becomes a central figure in the Earthnoid resistance. The story explores themes of identity, cultural clash, and the cyclical nature of war, as both sides grapple with their shared history and the consequences of their actions.

The narrative progresses through a series of battles, political maneuvering, and personal struggles, culminating in a climactic confrontation that forces both the Moonrace and Earthnoids to confront the true cost of their conflict. The Turn A Gundam, with its enigmatic origins and unparalleled power, serves as a catalyst for change, challenging the characters to reconsider their beliefs and forge a path toward coexistence. The story concludes with a resolution that emphasizes the possibility of reconciliation and the enduring resilience of humanity.