In 2039, the severe consequences of climate change forced sea levels to climb, erasing vast stretches of coastline. To make sure people heed the warning, a squadron of formidable warships armed with cutting‑edge weaponry appeared, crushing naval forces everywhere. Dubbed the Fog Fleet, each vessel carries a "Mental Model," a humanoid persona that embodies its core engine, the Union Core. Though each model displays a distinct character, all obey an unseen command called the Admiralty Code, which has erected a maritime blockade to stop human passage across the oceans.
Seventeen years later, Gunzou Chihaya, a graduate of the National Marine Academy, encounters the I‑401 and its Mental Model, Iona, who inexplicably allies with humanity. With this unexpected beacon of hope, Gunzou and his companions create a band of corsairs named Blue Steel and embark on a perilous voyage.
Sanzigen Japan