In a distant era where most humans have departed Earth, the Galactic Alliance of Humanity emerges to oversee exploration and safeguard mankind's future. Yet, a grave menace in the form of enigmatic creatures known as Hideauze sparks a cosmic conflict to avert humanity's demise. Equipped with Chamber, an independent robot, 16-year-old officer Ledo from the Galactic Alliance plunges into the war against these foes. However, a twist of fate sends Ledo adrift into space after losing control in battle, leading to a crash landing on a water-drenched Earth. On this cerulean world, the scavenger fleet Gargantia stumbles upon Chamber in the depths of the sea, believing it to be a valuable find. Misinterpreting their intentions as hostile, Ledo infiltrates Gargantia and seizes a young messenger named Amy, only to discover that the inhabitants are far from the threats he envisioned. Cut off from his space comrades and grappling with uncertainty, Ledo endeavors to adapt to his unfamiliar surroundings. Yet, his tranquility is fleeting, for the oceanic planet harbors secrets beyond his imagination.