The seabed── Despite being a part of this earth, it is still an unknown world full of mysteries. It all starts on a certain summer vacation. While traveling to the south by a ship, they suddenly get caught in a big storm. The protagonist Haruki, his sister Minamo, and his childhood friend Natsumi, all fall into the raging sea. When Haruki, who has already accepted death, regains his consciousness once again── He finds himself on a luxurious bed. "Not even the sunlight can reach here, this place is 5,000 meters deep. We are the undersea people, and this place is the underwater city Atalancia." A maid with animal ears, Rosania informs him unenthusiastically. And, as if to add another blow to the already surprised Haruki. "The one who brought Haruki-sama to Atalancia, it was none other than me." The Princess of Atalancia, Irionela