SERENA ROBINSON and TOBI DOYLE have been friends for sixteen years. Living next door to each other along the Ainsley River in South Carolina, they built sandcastles, played pirate ship, and collected shells virtually every day until a freak accident at the Robinson Phosphate Mine Company takes away someone near and dear to Tobi's heart. From then on, Tobi's mother, Mrs. Doyle, blames Serena's father, Mr. Robinson and his thirst for money for her husband's death. Serena and Tobi are torn apart by their parents' animosities, and the lifelong friends must go their separate ways in a town devastated by a recent war.

When Serena and Tobi witness the neighborhood apothecary DR. NATHAN TRASK lifting a limp body from his fishing boat, they don't realize they will be forced back together in a life-or-death effort to save the mermaid, MARI-MORGAN, from Dr. Trask's greedy plans. Serena has tried for years to convince Tobi that mermaids are real-her nanny ROSIE told her so-but he doesn't believe. However, Tobi discovers that not only do mermaids exist, but they have magical charms that are almost impossible to resist.

Serena may lose her best friend unless she can return the bewitching merwoman to the sea before she takes Tobi's heart and soul to the bottom of the ocean.