After humanity lost much of its land due to rising sea levels caused by global warming, the Fleet of Fog established dominion over the high seas.

Now the captain of I-401, Gunzou sets course for Yokosuka after accepting a mission from the Japanese government.

I-401 enters the Port of Yokosuka to receive a sample of the top-secret warhead developed by the Japanese government.

Yokosuka is now under attack by the fast battleships Kirishima and Haruna; I-401 sets sail to intercept them within the defensive barrier.

Defeated by I-401, Haruna has managed to retain her mental model form, but only Kirishima's union core has survived.

As Haruna and Kirishima attempt to befriend Makie, Kamikage fears that Makie may fall into the Fog's hands and orders a special forces assault.