Humans have left Earth and have made a paradise in space, Avalon, as humanity's new home and created the Galactic Alliance of Humanity to expand their journey through it. A threat known as Hideauze appeared before the Alliance and a war rages between them. Second Lieutenant Ledo and his A.I. Robot Partner Chamber joins the latest attack against the Hideauze. The attack ultimately ends in failure and a retreat order is issued, but Ledo is unable to return and is cast out into space. 6 months later, the salvage ship Gargantia retrieves Chamber, but are unable to take it apart. After everyone leaves, Ledo decides to take a look around and gather intel on where he is, but is soon chased and kidnaps one of the crew, Amy. He is chased outside to find out he is on a habitable planet, which shocks him enough to release Amy. The crew pins him on a pylon, and he calls Chamber, which the entire crew is surprised to see moving and floating.

Deciding to try and open a dialogue with these people, Ledo stays pinned to the pylon. They ask to talk to the Alliance in charge, but the crew have no idea what he is talking about and they believe that Chamber, who is translating, has another person inside. As Ridget, the commander, and the crew try to decide what to do, Amy gives Red food and begins talking. She explains that this is Earth, and when it was frozen over, some humans escaped to the stars while others stayed. Eventually, the ice melted and became a vast ocean, in which many people form fleets to salvage old technology from below. Looking for other items advanced technology, Ridget sends Bellows to the point where Chamber and Ledo Were found, and her ship is attacked by pirates. Amy asks Ledo to help them, and he agrees with the intention of earning their trust. Ledo boards Chamber and efficiently eliminate all enemies with no harm to Bellows' ship and crew, as Amy and the others watch in awe at its destructive power.

After Chamber wiped out all of the pirates that were threatening Bellows and her people, Ledo is perceived as a menace and kept under guard by the crew of the Gargantia. And perhaps rightfully so, as the pirate armada that is Empress Lukkage's fleet is racing towards the Gargantia Fleet with bloody revenge in mind...

In order to compensate for the damage to the hangar they caused, Chamber is put to work hauling around heavy cargo, but that leaves Ledo on the sidelines with nothing to do. Amy takes him around the fleet and introduces Ledo to the knowledgeable Dr. Oldham as well as her sickly but wise younger brother, Bebel.

Amy tries to help Ledo find a job and introduces him to a number of different industries aboard Gargantia, but he's just not able to land one. Then the wind dies down and the waves cease, making it a "Calm Day", during which time the fleet shuts down so a lot of repair work can be done. But for non-repair-related personnel, it's time to take a breather. Pinion invites Ledo, Amy and her friends to a BBQ party. Everything seems to be going smoothly...

Ledo decides to work as a fisherman, but he finds himself unable to pilot a regular Yunboro and sends Chamber to assist the other fishermen instead. Pinion then invites Ledo for lunch and offers him a position at his crew, but Bellows appears and also offers him a job at her salvage team. In the occasion, Amy and her friends dance to the crowd and Ledo mistakes a cooked octopus for a Hideauze and is about to shoot it until the others assure him that it is not an enemy, but food instead. Chamber, who finds himself causing a mess among the fishermen asks for Ledo's help and he uses his mecha to properly help the workers have a nice catch. Later in the night, Amy spends some time alone with Ledo, and a lighting puts him in state of alert and he arms himself to defend her, until she calms him down and dances to him upon his request.

When Ledo has an encounter with a whalesquid undersea, he attacks and kills it because of its similar appearance to the Hideauze. Little did he know that whalesquid are considered sacred, so murdering one is taboo, and soon the whole ship is in an uproar over it. Ledo, however, believes that whalesquid are Hideauze and he's determined to carry out his mission by slaughtering more of them before they decide to target humans on Earth. Just as Amy tries to stop Ledo, a huge pod of whalesquid approaches the fleet...

On his deathbed, Fleet Commander Fairlock entrusts Ridget with Gargantia. Meanwhile, Chamber finally gets a fix on the location of the Galactic Alliance, but they're so far away that Ledo will never be able to get home. As preparations are underway for Fairlock's funeral, Shipmaster Flange announces that he's leaving Gargantia. Ridget struggles to keep other Shipmasters from following suit. Pinion has plans of pillaging the ocean floor, so he joins the like-minded Flange. Amy is sad when two people she cares about are leaving as well, Melty and Ledo...

Ledo and Chamber go deep sea-diving to exterminate Earth's Hideauze.

With all the Hideauze in the area killed, Pinion and his crew manage to salvage ancient technology not seen anywhere, but instead of sharing it with the rest of mankind as originally planned, Pinion claims that they have the right to keep it to themselves, and sends a message to the other fleets warning them to keep out of their territory. As the message reaches the Gargantia, the fleet had just reorganized itself and there is the concern that other ships may leave to join Pinion's side as well. With the technology salvaged by them, Pinion's fleet defeats a hostile pirate crew who in return asks for a place among them, and he agrees. Ledo, still in shock upon learning the truth about the Hideauze hears from Chamber that the lightbugs that mankind use as power source were also created by the Hideauze as well which further proves their discovery.