Arpeggio of Blue Steel: Ars Nova

By 2039, global warming had caused sea levels to rise and a great deal of territory to be lost. As if in response, a mysterious group of warships cloaked in mist, "the Fleet of Mist," appeared in every corner of the ocean and began attacking human ships. Even though humanity gathered all its strength, it was utterly defeated by the Mist's overwhelming power. All of humanity's trade routes were blockaded by the Fleet of Mist, their political economy was destroyed, and the human race was steadily worn down. Seven years later, the Fleet of Mist's submarine I-401 appears before cadet Gunzo Chihaya. The humanoid life form that pilots the sub, who should be their enemy, instead offers her services to mankind.