Yuta is a humble fisherman in the late 1400s, who's life, by all indications is unremarkable. One evening a friend and fellow fisherman offers Yuta and another man a chunk of raw meat, the flesh of a mermaid. Each man samples the meat... one dies an immediate and painful death, while the other is transformed into a monster, a Lost Soul. Only Yuta appears to be unaffected by the mermaid's flesh. As the years pass, Yuta soon realizes that he is no longer aging, and his wounds miraculously heal. Yuta has become an immortal.

Over the course of 500 years, Yuta travels alone. meeting others who have had their lives ruined in the pursuit of the mermaid's flesh. Yuta continues to live and never age while watching his loved ones grow old and die- after many centuries Yuta realizes his immortality is no blessing, but rather a curse.

In the 1980s he finally meets another girl, Mana, who has successfully become an immortal, the only other person up to that point that had survived the brutal transformation of the mermaid flesh as he had. Together with Mana, Yuta's immortality seems less of a burden and the two set off to find a cure to the immortality so that they can grow old and die together.