Aoki Hagane no Arpeggio: Ars Nova is an anime series set in a future where rising sea levels have engulfed much of the land. Humanity is under siege from a mysterious fleet of powerful sentient warships known as the 'Fleet of Fog.' The story follows Gunzō Chihaya and his crew aboard the Fog submarine I-401 as they attempt to turn the tide against this formidable enemy using advanced technology and tactical prowess.