Soldier Ledo crash-lands on a flooded, unfamiliar Earth after a battle against the alien Hideauze scatters him across space and time, arriving with only his mecha partner Chamber for support. Taken in by the Gargantia fleet, a floating community that salvages the ocean's ruins to survive, Ledo starts learning an entirely different way of life from messenger girl Amy, who becomes his guide through a culture built on cooperation rather than combat.