Following the morning scuba dive, a pod of wild dolphins cavort around the boat and all fourteen divers eagerly re-enter the water for a wild-animal experience. One woman soon realizes she has not removed her weight belt and fights to stay afloat. She struggles to return to the boat ladder. A last, fateful lung places her directly beneath it, one arm upraised. The boat rises on a lazy swell and drops sharply into the trough, the lowest rung snapper her forearm, the stern striking her head. Her body is driven deep below the surface--unconscious.
Sometime later, her eyes fly open and she automatically kicks and fights to reach a surface that is now hopelessly distant. Suddenly she realizes she is breathing comfortably and stops to consider the situation. Once again her heart beats a wild rhythm when she spots a shadow in the distance, drawing closer, and finds herself face-to-face with a beautiful mermaid. As they scrutinize each other, a second, much younger mermaid joins them.
The two help the injured diver to a huge cavern inhabited by the Sirene, a society of mermaids, mermen, and their merkids. Unable to recall her name, the mermaids call her Serena and invite her to stay until her injuries heal. She falls in love with the kind and handsome sirene doctor but a rival for his affections threatens her happiness and, indeed, her very life. Serena finds that mermaids, although graceful and beautiful, can be extremely ruthless. Serena and Nathan leave the cavern and strike out on their own--a dangerous endeavor since there is safety in numbers and much risk for only two.
A catastrophic event changes everything and Serena faces an uncertain future.