Arpeggio of Blue Steel contains episodes 1-12.

In Arpeggio of Blue Steel, the Fleet of Fog came on the rising tides of the oceans. Where the waters swallowed the land, the ships of war, bearing far superior technology to anything humanity could produce, brought desolation to the armies of man.

Conquering the oceans, the Fog isolated the nations from one another. With no means of communication, and no ability to conduct trade, humanity was doomed, unable to inflict the slightest scratch upon the overwhelming power of the superior ships.

But not everything is lost. Despite the rumors surrounding his father, Chihaya Gunzou is following in his old man's foot steps by attending a naval academy. But while his father might be dead, or worse, defected to the Fog, Chihaya wants to gain the power to break the stagnation gripping humanity. And as it so happens, he gains that power when a mysterious girl approaches him, and claims she has orders to obey him.

As strange as it seems, the truth turns out to be even stranger, for that girl is Iona--that is, I-401 Iona, the Blue Steel, a submarine formerly with the Fog. Now Chihaya suddenly has the power he needs... even if he has to make an enemy of humanity to affect the change he desires!