August 1st, 2033- The deep sea pleasure cruise, , with over 50 passengers on board, sinks. And four days later— “There are two survivors. One of them is Hitose Katagiri, a grad student.” “And the other one is…” The completely forgotten “other survivor.” What happened in the four days until they were rescued? What lurks in the depths of the sea? And… who is the survivor? A closed off submarine, and open mysteries. This is a story with no set answer.