Rikki Chadwick, Emma Gilbert, and Cleo Sertori are three teenage 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 passes overhead, bathing the pool in light. The girls are rescued and brought back to shore where they return to their normal lives, but soon discover their lives have become anything but normal. The girls find out that seconds after contact with water, they transform into mermaids. After further experimentation, the girls also discover they have supernatural powers over water. The trio enlist the help of Cleo's friend Lewis McCartney to help them keep their secret.
A full moon in conjunction with a rare planetary alignment calls the girls to the moon pool at Mako Island where their powers and abilities are greatly augmented and strengthened to higher power levels. The second season revolves around the arrival of a new girl, Charlotte Watsford, who moves into the area and develops an instant attraction to Lewis. After Charlotte sees a film of her grandmother Gracie as a mermaid she meets Max Hamilton who was once romantically involved with Gracie. Happy to have met his lover's granddaughter he tells her about the previous mermaids and of Mako Island. Charlotte travels to the moon pool during a full moon and becomes the fourth mermaid in possession of all the girls' powers and abilities.
It's nearly the end of the holidays and Cleo and Rikki are about to face their last year of high school without Emma, who has gone overseas with her family. Together with Lewis, they're confident they know everything there is to know about Mako Island, especially now that they no longer have to worry about the effect of the full moon. But when water attacks the girls on a full moon, they realize a new, potentially dangerous force is developing on the mysterious island, and a boy with freakish free-diving skills who stumbles upon the Moon Pool could hold the key to understanding the powerful energy. Thankfully they find an ally in Bella, a new girl in town who reveals an equally surprising twist: she too is a mermaid with extraordinary super powers. Together, the new trio face phenomenal challenges as they discover the dark secrets of Mako Island and realize their destiny as mermaids.