In a dystopian world where the dark secrets of science and nature have led to unusual experiments, a strange series of disappearances begins to plague a quiet Japanese town. A group of friends, intrigued by rumors surrounding an old sewage system, decide to investigate what lies beneath the streets. However, what initially seems like an exciting adventure soon turns into a nightmare.
As they explore the depths of the sewer, the young people discover a secret laboratory and a macabre experiment: a mermaid, trapped in a web of conduits and subjected to horrific tests by a deranged scientist who believes he has found the key to immortality. As they venture deeper into this underground world, the group encounters unimaginable horrors, from mutations to scientific madness. Every decision they make brings them closer to a terrifying fate.
As they struggle to escape and free the mermaid, loyalties are tested, and dark secrets from each person's past are revealed. As tensions rise and time runs out, they must decide whether to risk their lives for a being that represents the fusion of the human and the marine. The fight for salvation becomes an epic journey that challenges the boundaries of morality, friendship, and the very nature of humanity.