All oceans have evaporated. Massive aquatic creatures now ride air currents through the sky, and the empire of Daiichia, a capitalist superstate, stakes its national pride on harvesting giant crabs from these airborne waters. The crab canning ships that carry out the hunt are floating hells: bottom-tier laborers, sold into debt by orphanages or driven by desperation, work brutal shifts for meager rations under the constant threat of death.

Ruka and Hiiragi were raised together in an orphanage before being sold to one such ship. They dream of paying off their debt and escaping together, but the ship's overseer, Kujo, enforces a reign of sadistic cruelty. When Ruka defies her, Hiiragi shields him and is executed on the spot. The murder shatters Ruka's despair and hardens his resolve: he will survive, repay his debt, and leave the ship alive.

The crab fishery demands more than guts. Workers inject a muscle-enhancing drug to wrestle the giant crabs in hand‑to‑hand combat, snapping their joints in a brutal form of submission wrestling. Death is routine. Kujo, a cold-eyed beauty with an eye patch, singles out Ruka for special torment, forcing him into ever more dangerous encounters. Ruka barely survives with the help of fellow workers: Daemon, a Mohawked enforcer with rebel blood; Tomura, an alcoholic shell‑peeler who eventually sacrifices himself; and the mysterious Yuuri, a newcomer from a crashed ship who carries a hidden agenda.

After a riot by communist sympathizers—the "Reds"—disrupts the ship, Ruka and his allies are transferred to another vessel, the Uguisumaru. There they find a transformed ship: gambling, drugs, and factional leaders now rule the crew. Yuuri pushes Ruka to become the figurehead of a labor union, while the idealistic student Tajima, who boarded to document the workers' plight, is broken by the ship's sexual exploitation and commits suicide.

Throughout the story, the desperate labor of the workers is juxtaposed with the luxury of the floating upper‑class settlements that drift above the clouds. The narrative follows Ruka's journey from suicidal resignation to defiant leadership, culminating in the formation of a workers' union and a final, all‑out revolt against the captain and the system that grinds them down.