Rikki Chadwick, Emma Gilbert, and Cleo Sertori are three sixteen-year-old Australian girls who find themselves stranded on the mysterious Mako Island. They eventually find themselves in a pool under a dormant volcano just as a full moon appears overhead, bathing the pool in light. The girls are rescued and return to normal life, but soon discover their lives have become anything but normal: seconds after contact with water, they transform into mermaids with supernatural powers over water. The trio enlist the help of Cleos friend Lewis McCartney, who attempts to discover the secret behind their transformations. Everyday situations, such as bathing and rain, become tricky as the girls struggle to cope with their newfound abilities, which have many advantages and disadvantages, while also trying to keep them a secret from everyone else, including their families. They soon adapt to their new abilities and lifestyles.