"Mermaid Forest" is a dark fantasy manga series that explores themes of immortality, human desire, and the consequences of seeking eternal life. The story centers on Yuta, a man who gained immortality after consuming mermaid flesh centuries ago. His quest to find a cure for his condition leads him to encounter others who have also consumed mermaid flesh, often with tragic or grotesque results. Along the way, he meets Mana, a young woman who was raised in isolation and also bears the effects of mermaid flesh. Together, they navigate a world filled with danger, betrayal, and the lingering remnants of mermaid legends.

The narrative is episodic, with each arc delving into different characters and settings tied to the myth of mermaids. In one story, Yuta and Mana encounter a village where mermaid flesh is used to create immortal beings, only to discover the horrifying price of such power. Another arc explores a family cursed by their ancestor's consumption of mermaid flesh, leading to generations of suffering. The series frequently examines the moral and emotional toll of immortality, as characters grapple with loneliness, loss, and the erosion of their humanity.

Yuta is portrayed as a weary, compassionate figure burdened by his long life and the memories of those he has lost. Mana, in contrast, is innocent and curious, providing a counterbalance to Yuta's world-weariness. Their relationship evolves as they face various threats, including other immortals who exploit their powers for selfish or destructive purposes. The series also features recurring antagonists, such as the enigmatic and malevolent Nae, who seeks to manipulate mermaid flesh for her own ends.

The manga blends horror, mystery, and folklore, creating a haunting atmosphere that underscores the tragic nature of its characters' struggles. The mermaid mythos is central to the narrative, with the creatures depicted as both alluring and terrifying, their flesh granting life but at a terrible cost. The series' exploration of immortality is nuanced, highlighting the physical and psychological toll it takes on those who seek it, as well as the societal and cultural implications of such power. Through its interconnected stories, "Mermaid Forest" presents a compelling and often unsettling examination of human ambition and the price of eternal life.