In the near future, rising sea levels have submerged most of the land and human habitats. Ikaruga Natsuki lost his family and his leg a few years ago. He decides to find his old home in a small seaside town, which is now mostly underwater. All he has left is a small ship and a submarine inherited from his grandmother, a renowned oceanologist. His hope for a normal life is spurred by a somewhat suspicious debt collector, Catherine. Natsuki’s task is to repair the submarine and prepare it for a journey to the ocean floor to retrieve valuable items left behind by his recently deceased grandmother.