Arpeggio of Blue Steel (蒼き鋼のアルペジオ, Aoki Hagane no Arupejio) is a Japanese manga series produced by Ark Performance and serialized in Shōnen Gahosha's Young King Ours. Twelve manga volumes have been released and an anime series by Studio Sanzigen aired from October 7, 2013 to December 23, 2013. Two films based on the series were released respectively on January 31 and October 3, 2015.
Due to global warming and rising sea levels in the early 21st century, much of Earth's landmass has been lost. In 2039, fleets of powerful sentient warships, armed with advanced technology and possibly 'alien' weaponry, mysteriously appear and devastate the world's naval forces. These ships, collectively called as 'The Fleet of Fog', impose a worldwide naval and aerial blockade, preventing humanity from both traveling the oceans and to other nations. During the blockade, the Fleet of Fog created Mental Models, humanoid avatars containing a ship's Union core, as a means to develop self cultivation and to overcome their lack of creative thinking of tactics that humans possessed, which at the same time made the Mental Models develop their own unique personality.
In 2056, 17 years after the blockade began, Gunzō Chihaya, a former student of the Japanese National Marine Academy, is the captain of a small group of privateers called the 'Blue Steel'. The Blue Steel are infamous for possessing a Fog submarine, the I-401, along with its Mental Model Iona, who defected to the human side. Due to I-401's technology and Gunzō's tactical skills, the Blue Steel have overcome many encounters with the Fleet of Fog and enemy governments and factions with their own agendas. Along the way they will also gain allies of their own from both sides, increasing their chances against seemingly overwhelming odds.