Three young girls accidentally stumble on a tidepool on a secret island; when bathed in moonlight, the tidepool transforms them into mermaids, with great powers over water--but also a tendency to sprout a tail whenever they come into contact with water.
The original three mermaids--Emma, Cleo, and Rikki--separate at the end of season two, when the actress playing Emma left the show. In season three, another character, Bella, is added: Bella had become a mermaid in an unrelated incident some years earlier.
The mermaids also face various antagonists, ranging from rivals (e.g., the antagonist for season two, Charlotte, who manages to obtain similar powers from the moon pool) to those who wish to exploit the mermaids for their own profit.
As with Jonathan M. Shiff's other productions, the program is aimed largely at young teen audiences.