According to legend, eating the flesh of a mermaid grants one immortality. But the truth is, only a tiny proportion of the people who try mermaid flesh actually become immortal. Most either die from it or are changed into monsters called "lost souls". Yuta and Mana are two of the "lucky" ones. Yuta is a 500 year old former fisherman and Mana is his new travelling companion, a teenaged girl Yuta met shortly after she became immortal. Together they journey all across Japan looking for a way to become "normal" again.