Humanity has lost much of its developed land due to global warming in the first half of the 21st century. Then, a “Fleet of Fog” loaded with super weapons suddenly appears all over the world. Without the ability to withstand this fleet, humanity was defeated and could no longer travel the seas. 17 years after the devastating naval war, Gunzou Chihaya and his crewmates somehow commandeer a “Fleet of Fog” submarine named I401. Together with Iona (I401’s “mental model” or incarnation in humanoid form), they take on the “Fleet of Fog.”