"Overman King Gainer" is set in a distant future where humanity has migrated to domed cities in Siberia due to environmental collapse. The story unfolds in a world dominated by the Siberian Railroad, a powerful organization that controls transportation and resources. The narrative centers on Gainer Sanga, a young man living in the domed city of Shangri-La, who becomes entangled in a rebellion against the oppressive Siberian Railroad. Gainer is a skilled gamer, and his expertise in virtual simulations proves crucial when he accidentally pilots an Overman, a highly advanced humanoid mecha, during an escape attempt.

The Overman, named King Gainer, is a unique machine with extraordinary capabilities, including the ability to manipulate gravity and perform feats beyond conventional technology. Gainer teams up with Gain Bijou, a mysterious and charismatic leader of the Exodus movement, which seeks to liberate people from the domed cities and establish a new society. Together, they embark on a journey across the Siberian wilderness, pursued by the Siberian Railroad's forces, including elite Overman pilots like Adette Kistler and Asuham Boone.

As the story progresses, Gainer and Gain face numerous challenges, including battles with rival Overmen, internal conflicts within the Exodus group, and the harsh realities of the outside world. The narrative explores themes of freedom, survival, and the consequences of humanity's reliance on technology. Gainer's growth as a pilot and leader is a central focus, as he learns to harness the full potential of King Gainer while grappling with the moral complexities of their mission.

The supporting cast includes characters like Sara Kodama, a skilled mechanic and Gainer's childhood friend, and Biel Oum, a young boy who idolizes Gainer and becomes a key member of the Exodus group. The antagonists, such as the ruthless Siberian Railroad commander Cybela Qwenthur, provide formidable opposition, driving the tension and stakes of the story. The series blends action, drama, and science fiction elements, creating a compelling narrative that balances intense mecha battles with character-driven storytelling.