Taking place in the outermost reaches of the galaxy in a futuristic period, this story commences during a time when the Human Galactic Alliance fights an incessant battle for their survival against a monstrous and unsightly group known as the "Hidiaazu." During a particularly intense skirmish, the youthful Lieutenant Redo, accompanied by his humanoid weapon, Chamber, experiences a bizarre interruption of both space and time. Upon awakening from his induced slumber, Redo discovers himself standing on Earth, which represents the final frontier.
This planet is entirely submerged underwater and inhabited by humans who reside in immense fleets of ships, scouring the abyss in pursuit of invaluable relics necessary for their survival. Redo finds himself on one of these fleets, named the Gargantia; however, without any knowledge of the planet's past or culture, he is forced to live amongst the Gargantia's inhabitants, including a young messenger named Amy, who's only fifteen. As someone who has only ever experienced the lifestyle of a soldier, Redo struggles to comprehend the idea of living peacefully for days on end, and this becomes just one of several epiphanies during his stay on Earth.
