Arpeggio of Blue Steel

During the mid-21st century, humanity lost much of its terrestrial lands due to the rise in sea levels caused by global warming. As if summoned by the ensuing chaos, the Fleet of Fog appeared and established dominion over the high seas. Humanity seemed doomed to suffer a slow death.

Now the captain of I-401, Gunzou sets course for Yokosuka after accepting a mission from the Japanese government. Takao, a Fleet of Fog Heavy Cruiser, stands in his way. As Takao steadily chips away at I-401's decoys and threatens the boat itself, Gunzou decides that a showdown is necessary.

Triumphant over Takao, I-401 enters the Port of Yokosuka to receive a sample of the top-secret warhead developed by the Japanese government. However, a powerful politician demands that Iona fall back under government custody. As the tension mounts, Yokosuka is suddenly subjected to cannon fire!

Yokosuka is now under attack by the fast battleships Kirishima and Haruna. I-401 sets sail to intercept them within the defensive barrier, using every trick in their strategic arsenal to evade the raw firepower of the two battleships. Sensing the changes humans have wrought on I-401's tactics, Kirishima and Haruna decide to take her out with an all-out attack, but Gunzou still has a trump car...

Defeated by I-401, Haruna has managed to retain her mental model form, but only Kirishima's union core has survived. Taken in by a mysterious girl named Osakabe Makie, they find that her father is the developer of the vibration warhead and decide to retrieve his files, but...

Haruna and Kirishima have been asked by Dr. Osakabe to become friends with Makie. However, fearing that Makie may fall into the Fog's hands, Assistant Secretary Kamikage has ordered an assault by special forces. Haruna tries to protect Makie, but their tenuous friendship is jeopardized when Makie learns of Haruna's true nature...

I-401 heads to her secret home port with Makie, Haruna, and Kirishima on board. There, they find the heavy cruiser, Takao, waiting for them. As the crew prepare for their upcoming journey to the United States with repairs and supplies, they and the mental models learn a lot about each other...

Iwo Jima has been surrounded by the Fleet of Fog's Eastern Patrol Fleet 1. Gunzou invites the Mental Model of its flag ship, Kongou, ashore to the island to speak with her. Unable to accept the changes occurring to members of the Fog, Kongou blames Chihaya Gunzou and makes a critical decision...

As missiles rain down on Iwo Jima, the Mental Models work together to withstand the attack from the Fog's East Patrol Fleet One. With their help, I-401 manages to break through the blockade, but Kongou is hot on their heels...

I-401 has been hit with two corrosive torpedoes in an ambush by Iona's sister ships, I-400 and I-402. 401's repair programs are impaired by a special program that had been brought in by the torpedoes. As 401 sinks and Gunzou weakens from his injuries, Iona is forced to make a choice...

Restored and strengthened by Takao's sacrifice, 401 forges on to America with 400 and 402 in pursuit. The sister ships face off in the waters of Hawaii. With enhanced specs, five Mental Models, and Gunzou's tactical prowess, the twin submarines are no match for the new 401... but Iona wishes them no harm.

Losing Maya has made Kongou go berserk. Absorbing Maya and light cruisers has given Kongou unprecedented power, which she uses to obliterate the Fog's U.S Pacific Fleet and now turns on 401. To save Kongou, Iona decides to go in alone...