Mermaid Forest is about an ancient Japanese legend says that if one were to partake of the flesh of the mermaid, one would be granted eternal life and youth. Yuta ate the mermaid's flesh 500 years ago and became immortal. Since then he has had to deal with living a lonely never-ending life, watching his loved ones grow old and die.
Wishing to have a normal life, to live and to die like others, he searches for a mermaid for centuries, thinking she may be able to tell him a way to become mortal again. When he arrives a village, he meets a girl named Mana, who also became immortal by eating a mermaid's flesh. Together, they start their journey in attempting to find a mermaid and regain their humanity.