Castaway.Humans have left Earth to make a paradise in space named Avalon as humanity's new home, creating the Galactic Alliance of Humankind to expand their journey through it. Mollusk-like lifeforms known as Hideauze appeared as a threat before the Galactic Alliance, and a war rages between them. Ensign Ledo and his Machine Caliber Chamber K6821, join the latest attack against the Hideauze. The attack eventually ends in failure and a retreat order is issued by Ledo's superior, Kugel. However, Ledo is unable to return, and he is cast out into space. Six months later, the crew members of the salvage ship Gargantia retrieve Chamber from the bottom of the ocean, but are unable to take it apart. After everyone leaves, Ledo decides to explore and examine his current location, only to be noticed by Pinion, head of the repairmen. Ledo kidnaps a courier named Amy, as he is chased to the outside and cornered on a pylon by the entire crew. He finds out that he is on a habitable planet, which shocks him enough to release Amy. Then, he summons Chamber, who confirms that this is the planet of humanity's origin.