In the early 21st century, the majority of Earth's landmass is submerged due to rising sea levels. Enter the Fleet of Fog, a host of sentient alien battleships of unknown origin who easily decimate earth's naval forces. Humanity is now on the verge of extinction. In the year 2039, the stalwart Captain Chihaya Gunzou and his intrepid crew on the Blue Steel fleet may be humanity's last chance to fight off the alien invaders. The Blue Steel fleet happens to contain a secret weapon that could turn the tide of war: a mysterious I-401 submarine that can transform into a girl.

"The mansion where Professor Osakabe and Makie live is under attact, and nobody is happy about it: not the government, not the residents, and not even the attacking soldiers, who find themselves face to face with Haruna and learn firsthand that even without their battleships the Fog's mental models are a force to be reckoned with! Haruna's world has been turned upside down since the I-401 destroyed her and Kirishima's battleships, She's learned that Osakabe Makie is a Design Child, and a potentially valuable resorce for the Fog, but Makie has also become her friend. And now Haruna is quickly learning to the meaning of both "friendship" and "loss" -- concepts she's never have to grapple with before" -- Page [4] of cover.