In a future where humanity has migrated to domed cities due to environmental collapse, society is divided into rigid classes, and movement between cities is heavily restricted. Gainer Sanga, a young gamer, becomes entangled in a rebellion after encountering Gain Bijou, a mysterious woman leading a group of refugees attempting to escape the oppressive system. Gainer inadvertently pilots an Overman, a highly advanced humanoid mecha, named King Gainer, which responds uniquely to his abilities. The Overman’s advanced AI and Gainer’s natural talent make them a formidable pair.

The refugees, known as the Exodus, aim to reach "Siberia," a mythical land of freedom outside the domed cities. Their journey is fraught with challenges as they face pursuit by the Dominion, the authoritarian government enforcing the class system. The Dominion deploys its own Overmen and military forces to stop the Exodus, leading to intense mecha battles. Gainer’s growth as a pilot and leader becomes central to the group’s survival, as he learns to harness King Gainer’s full potential while grappling with the moral complexities of their mission.

Key characters include Gain Bijou, whose enigmatic past and leadership drive the Exodus; Sara Kodama, a skilled mechanic and Gainer’s childhood friend; and Adette Kistler, a Dominion officer whose ideals clash with her duty. The series explores themes of freedom, societal control, and the human spirit’s resilience, set against a backdrop of high-stakes mecha action and strategic conflict. The narrative balances character development with plot progression, culminating in a climactic struggle that challenges the status quo and redefines the characters’ understanding of their world.