The setting and plot of Overman King Gainer are a loose adaptation of the novel series La Compagnie des glaces by the French writerGeorges-Jean Arnaud. After an environmental cataclysm, much of the world's population retreated into domed cities called Domepolis, which are run by an organization known as "London IMA" and its police arm, "Saint Regan". However, the Domepoli are maintained and supplied by privatized firms such as the "Siberian Railroad Company". Many years have passed since the cataclysm, and many have begun to believe that the world environment has recovered sufficiently enough for humans to begin living outside Domepoli. As such, some inhabitants begin planning Exoduses to leave these cities and to resettle their ancestral homelands. However, this does not sit well with the London IMA or the private firms that monopolize trade between the Domepoli, as the loss of people would not only lead to a loss in tax and trade revenue, but if the Exodus were to be successful, it would show the people that it was no longer necessary to rely on the London IMA or the private firms for survival. They fiercely oppose any Exoduses, spreading propaganda on the evils effects of Exoduses on the Earth's environment, and using military force to prevent any attempted Exodus. The story begins in a Domepolis in Siberia, where championship video game player Gainer Sanga is arrested by Siberian Railroad policewoman Adette Kistler on suspicion of being an Exodus member. Ironically he isn't, but his friends Sara Kodama and Bello Korissha along with his schoolteacher Mamado Azaf are members of the Gauli team, a militia unit of the local Exodus group. At the same time, Exodus expert and coordinator Gain Bijou lets himself be arrested by the Siberian Railroad police as part of his plan to infiltrate the city and steal an "Overman", which is a biomechanical giant robot, for use in defending the Exodus. Gain is placed in the same prison cell as Gainer, and when Gain initiates his escape, only Gainer is willing to escape as well. They infiltrate the castle of Duke Medaiyu and steal an Overman in the Duke's secret museum collection, which Gainer logs into as his videogame handle: "King Gainer". As Gainer gets the hang of piloting a real Overman, he and Gain encounter shut-in Princess Anna, the Duke's daughter, who supports the Exodus and wants to see the world and people other than her tutor, Lioubov Smettana. Using an annual festival presented by idol singer and co-Exodus leader, Meeya Laujin, as cover, the Exodus executes their plan: take much of the Domepolis, block by block, using heavy hauler machines called Silhouette Mammoths and move them over 3000 km across the Siberian tundra to their ancestral homeland, "Yapan". As the Exodus moves out, the Siberian Railroad police chief, Yassaba Jin, mobilizes his forces to stop them. Other obstacles to the Exodus are Kistler's and Jin's subordinates Jaboli Mariela, Kejinan Datto, and Enge Gam, Siberian Railroad president Kizz Munt, Saint Regan policemen Asuham Boone and Zakki Bronco, and Overman aces Cynthia Lane and Kashmir Valle.
Gainer is the headstrong main protagonist of the series. He lived his days as a high school truant and online gaming champion before meeting Gain Bijou, an Exodus specialist under the captivity of the Siberian Railway forces. Gainer initially opposes the Yapan Exodus, but through a series of coincidences and his connections with Gain and Sara, he suddenly finds himself soldiering for their cause. He pilots the long-haired Overman dubbed 'King Gainer'. ;Sara Kodama
Sara Kodama is a diligent high school student and member of the Gauli Squad, a rebel soldier faction which supports the Yapan Exodus. Her work ethic is matched only by her heart, which she devotes mainly to Gainer. Her plan upon completion of the Yapan Exodus is to cultivate a farm. ;Gain Bijou
A self-proclaimed 'Exodus specialist', Gain is an expert sniper and important member of the Yapan Exodus. His nickname, 'Black Southern Cross', was earned by his reputation for leaving bullet wounds in the shape of a cross within his fallen enemies. His affable demeanor outside the battlefield is pierced upon discussion of his mysterious past. ;Princess Ana Medaiyu
Princess Ana is the daughter of Duke Medaiyu. Her lively nature and insatiable curiosity lead her to join the Yapan Exodus under the guise of a hostage. She owns three intelligent pet squirrels that heed her every command. ;Lioubov Smettana
Adette debuts as a commander of the Siberian Railway forces, and infiltrates the traveling city units in order to capture Princess Ana. Gain quickly thwarts her attempt, and with nowhere else to go, she begrudgingly starts a new life on the Exodus as a high school teacher for Gainer and Sara's class. She's a tough and shrewd woman, but childlike in her temper and affection toward men. ;Bello Korissha
A fellow Gauli Squad member and classmate of Gainer and Sara. He has feelings for Sara, but never seems to show them outside random acts of jealously toward Gainer. Despite this, he remains good friends with Gainer and Sara and a helpful ally on the battlefield. ;Meeya Laujin
Cona is the temperamental young mechanic who repairs the Silhouette machines and King Gainer. Early on, she competes with Lioubov for Gain's affection, but quickly moves on to capture the attention of Gain's former comrade. ;Nann
During her first appearance, Cynthia is simply an online gaming buddy of Gainer's hidden behind a virtual avatar. However, it is discovered later on that she is part of the Siberian Railroad forces, raised and favored by Kizz Munt. She is almost always eating candy and seems to treat piloting as a game, as well as dating. Her trademark whimsy and inability to take things seriously comes with major consequences, as she ends up hurting a Gainer later in the series during a friendly piloting competition between Overmen. Cynthia's natural piloting ability is said to have come from her mother, whose name is never mentioned; her only living relative appears to be Martina Lane. The shape-shifting Dominator is her Overman of choice. ;Yassaba Jin
Asuham is a recurring antagonist and member of the Saint Regan police force. His only real objective is to capture Gain and force him to confront his sister, Karin, who has conceived and raised Gain's child alone. He does so to the point of obsessiveness and madness until the very end of the series when Karin drags him home in an Overman, claiming to have no interest in Gain. ;Zakki Bronco
Karin is Asuham's sister and Gain's former lover. After a one-night stand, she ends up conceiving Gain's daughter; however, she has no lingering feelings for Gain and only appears to rescue Asuham. ;Martina Lane
