Arpeggio of Blue Steel - Ars Nova is set in a world where global warming has caused sea levels to rise dramatically. The human race, unable to compete with the advanced technology of the Fleet of Fog, a fleet of sentient battleships, is forced to retreat from the seas. Chihaya Gunzou and his crew aboard the I-401 submarine and their comrades wage a secret war against the Fleet of Fog in order to regain control of the seas.
The story follows Chihaya Gunzou and his crew as they fight against the powerful Fleet of Fog using their I-401 submarine. They are joined by other submarines such as Iona, Takao, Haruna, Kongo and Kirishima who have been given sentience by Mental Models (humanoid personifications) created by AIs contained within them. Together they battle against overwhelming odds in an effort to reclaim the seas for humanity. Along their journey they encounter various obstacles such as rogue Mental Models and enemy ships while also uncovering secrets about the origin of the Fleet of Fog.
