Marin and her younger sister Urin are sea-dwelling people. One day they find a beautiful silver ring near their home. Marin, wanting to return it to its owner, takes the hesitant Urin with her to the surface world. She does this despite her knowledge of the legend of the turtle old man who left the bottom of the sea and never returned.
After the death of his parents, Ryou and his sister Akane live together. Despite their difficult situation, they lead a relatively happy and normal life. Everything changes one day when they learn about a secret Japanese government project. Due to a declining birth rate, the Japanese government is testing a program in which two young people are forced to marry after having their personalities matched by a supercomputer.