Arpeggio of Blue Steel takes place in a near future in which much of the earth land mass has been lost due to rising sea levels, and an alien race, for reasons never explained, drive humans off the sea with an inhuman fleet of unbeatable and sentient ships called the Fleet of Fog. Humans throw everything they have at the Fleet of Fog and are incapable of so much as scratching them. The aliens also disrupt world-wide communications so that humans are confined to their own local continents and all sea travel and communication between countries is impossible.
The story starts with a teenage boy named Gunzou Chihaya in a military (naval) academy, bemoaning with his small circle of friends what the human race has been relegated to, and how useless the naval academy now is, etc. Gunzou is something of a loner, partly due to the fact that his father is rumored to be a traitor who went over to the Fleet of Fog.
At the beginning of the series, Gunzou is approached by a young girl who tells him she is there to follow his orders. It turns out the young girl is the human-seeming avatar (mental model) of the Fog Fleet submarine, I-401. Her name is Iona and she only knows that she has been sent to find Gunzou and to follow his orders and only his orders. It’s never stated in the series, but the implication is that somehow Gunzou’s missing father managed to turn her into a Fog Fleet renegade and send her to Gunzou. So Gunzou becomes captain of a Fog Fleet submarine and he and Iona and his friends begin their own private war against the Fleet of Fog.
The Fleet of Fog in turn cannot just let this renegade submarine go and so they send ships after her to sink her. Thanks to Gounzou’s tactical skills and Iona’s technology, Gunzou-tachi manage to survive and win these encounters. In addition, as each ship confronts and battles Iona-tachi, the female mental model of each ship is “corrupted” and joins Gonzou’s fleet. This is a harem anime essentially, but it's one with such a unique plot.
Although Gunzou-tachi basically work for themselves on their own agenda, picking up allies and making rescues along the way, one of the main story plots is the job they undertake for the Japanese government to deliver plans for a new superior weapon to the US. The US is the only nation left with the resources to manufacture this new weapon, which will allow all of humanity to fight back against the Fleet of Fog. Much of the series is the I-401’s battles to cross the Pacific despite the Fleet of Fog’s efforts to stop her.
Overall, I liked this series. I would be hard-pressed to say exactly why, since I’m not that fond of any of the individual elements here, like naval ships and battles or harem anime. And yet the series just appealed to me, perhaps also because it was short enough that it didn’t have extraneous episodes and I didn’t feel like I wasted time with it. And I probably have a soft spot for characters like Gunzou that have a strong moral code and do what’s right because it’s right, not for any other reason. I’m glad I watched it.