After a teenage delinquent is killed while saving a child's life from an approaching car, the rulers of the underworld send him back to become an "Underworld Detective" investigating demons apparitions in the human world.
5000 years ago, evil demigod Poseidon and his minions attack the good mermen. A dolphin saves one of them, baby Triton. Later, as a young boy, Triton decides to avenge his family and end the reign of evil Poseidon and his monsters.