A boy who came from the heaven encounters a city of ships, Gargantia. In the distant future a young boy, Ledo, is raised in space knowing nothing but war, until he comes to Earth, a planet whose surface is nearly covered in water, where he witness the sea the first time. On the vast seas with an aquamarine glow, people form huge fleets and live a humble, but happy life. Ledo is the only foreigner in this world with foreign customs and language, but he's not alone. What can he do to make a living here? What should he live for now? His days on this verdurous planet will make him face those questions.