To the Cradle in the Ocean In the near future, the world's sea levels have suddenly risen without explanation, flooding all of the coastal regions and slowly flooding more land every year. Natsuki Ikaruga is a young man trying to build a submarine so he can recover treasure from the now submerged home of his grandmother, marine geologist Nonko Yachigusa. After repairing the submarine, he sets out with his business partner Catherine and childhood friend Minamo Kamishiro. Exploring the home, Natsuki discovers a female Humanoid, a human-like android, named Atri. Natsuki reveals that Nonko, who was Atri's master, died a month prior due to illness, so Atri designates Natsuki as her new master...