Sixteen-year-old Ensign Ledo is a soldier in the Galactic Alliance, piloting a Machine Caliber (マシンキャリバー, Mashin Kyaribā), an artificial intelligence-automated, humanoid-shaped mecha, which he refers to as "Chamber".

When he awakens, he discovers that he and Chamber have been "salvaged" by a rag-tag band of "primitive" human scavengers aboard the Gargantia, a massive fleet of ships on an ocean-covered planet.

Chamber tells him that local star charts reveal the planet is Earth, the birthplace of humanity, a place that members of the Galactic Alliance were only familiar with from stories and believed was a myth.

Gen Urobuchi explained on the official website that the message of the story is aimed towards those in their teens and 20s, who are either about to enter into society or recently have, and is meant to cheer them on and to encourage them that "going out into the world isn't scary".

On March 30 and 31, 2013, at the Anime Contents Expo Bandai Visual's booth gave away 8000 copies of the first two episodes on Blu-ray Disc.

Murata had been harboring the desire to work on a story involving people living on ships atop a world of water about ten years before the series started.

Urobuchi was put in charge of writing the first and last episodes once the green-light was given for a thirteen-episode series in order to set the theme for the other writers.

On April 3, 2015, at an event in Tokyo, Bandai Visual revealed that a sequel for the anime was planned, but due to "various circumstances", was scrapped.

[24][25] A spin-off manga series, titled Gargantia on the Verdurous Planet: Mizuhana no Bellows (翠星のガルガンティア 水端のベローズ, lit.

"The Headwaters' Bellows"), drawn by Shū with story assistance by Hiroki Uchida, began serialization in Enterbrain's Famitsu Comic Clear website on June 7, 2013.