Emma, Cleo and Rikki are sixteen year old girls growing up on the sun-drenched beaches of the Gold Coast. Emma is confident, brilliant, responsible, and athletic, Cleo is slightly insecure, sweet, and awkward while Rikki is the aloof and rebellious new girl.

The girls find themselves stranded at sea one day, floating towards the mysterious Mako Island. The three girls explore the eerie jungle-covered island before becoming trapped in an ancient cavern under a volcano. Discovering an underwater channel, they decide to swim to safety. As they enter the pool, the light from a full moon illuminates the water, creating a mysterious glow.

Emma, Cleo and Rikki get off the island as fast as they can and back to "normal" life. However, the girls discover life is never going to be normal again; ten seconds after they touch water, they turn into mermaids. After further experimentation, they find they have supernatural powers; Emma can freeze water, Rikki can boil water and Cleo can manipulate the amount and shape of water.

The girls enlist the help of their friend Lewis, who considers himself a scientist, and he attempts to discover the secret behind their transformations.

Everyday situations turn tricky as the girls struggle to cope with their new-found powers, which have numerous advantages and disadvantages, while also trying to keep them a secret from everyone else.