For as long as she can remember, Charlie, a young mermaid has been led to believe that humans were heartless monsters who would only destroy her kind, the merfolk yet she's always been curious by the world above where the humans dwell. When she reaches her fifteenth year, she swims up to the surface for the first time and spends the day observing a human kingdom. during her exploration, she notices that the humans are not as monstrous as her parents made them out to be. Particularly the queen of this kingdom and her young son the prince.
However at night fall, her father the sea king conjures up a terrible storm that floods and destroys the kingdom. The only survivor is the prince who Charlie manages to save from drowning. The boy is dazed but a bond is formed between him and the mermaid, a bond so strong that his heart will never yearn for anyone save for her. While Charlie is so horrified by what her father is done and so fascinated and sympathetic by the humans she's seen that she becomes displeased with her life among merfolk.
She starts to pay visits to her grandfather, the only one other than herself who feels compassion for humans and learns more about them from him. Including that merfolk can grow legs on dry land and that humans have an immortal soul, allowing them to enter the kingdom of heaven when they die. Whereas merfolk like them live for three-hundred years but turn to sea foam once they die. Charlie wants to earn an immortal soul but her grandfather tells her that a mermaid can only earn a soul if she wins the love of a human.
Years go by and Charlie continues to learn more about humans. When she becomes an adult her father tells her that the time has come for her to full fill her duty and protect her people by using her beautiful, siren-like, voice to lure sailors to their deaths thus decreasing the human population more. But Charlie refuses and when her father insists that she has no choice, she swims away from home.
Unfortunately this leads to her getting captured by a cut-throat pirate crew; Husk the foul mouthed and alcoholic quartermaster, Rosie a twisted yet maternal woman from a temple who now serves as the crew's navigator and boatswain, Angel quick-thinking and sex driven gunner who was previously a petty thief, Fat Nuggets the powder monkey or rather powder pig, and Alastor the ruthless captain who has been notorious for hunting, killing, and devouring merfolk.
But when Charlie inadvertently reveals that she's the daughter of the sea king, he decides to keep her alive for awhile due to a personal vendetta he has against her father. So she is now being held prisoner along with Vaggie, the princess of an Amazon tribe who was sold into slavery and later captured from the slave ship by the pirates. The two decide to plan a secret escape but feelings that they develop for the crew members complicate their plans. Same could be said for Alastor when his revenge plan is complicated by growing feelings for Charlie.