Emma Gilbert, Cleo Sertori and Rikki Chadwick 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 Cleo's 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, including new rivalries.
Series two introduces Charlotte Watsford, a rival to Cleo after taking Lewis as her boyfriend, who eventually gains mermaid powers as well and becomes the villain of the show. However, she eventually loses both her powers and Lewis.
Series three sees the arrival of Bella Hartley, who has been a mermaid since the age of nine. The three become eventual friends and begin to get attacked by a mysterious tentacle who begins to send once more to Mako Island. They eventually learn that they are in the sights of a comet that had its sight on destroying the earth. Using their powers, they manage to stop the comet and go to graduation, where Lewis comes back with open arms to Cleo.