About the Arpeggio of Blue Steel Manga Volume 17 (Mature)

Seventeen years ago the Fleet of Fog took control of the oceans, barring humanity from traveling between continents or islands. Now, a weapon powerful enough to damage the Fog's vessels has finally been invented. The submarine I-401, with its humanoid avatar - a 'mental model' named Iona, who's taken the shape of a young girl - was once part of the Fleet of Fog, but has chosen to side with humanity. Captained by Chihaya Gunzo and his crew, the I-401 is the only ship under human control that can move freely through the seas. But she answers only to her captain, and he answers to no one.