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. Together with Lewis, they re confident they know everything there is to know about Mako Island. But when water attacks them 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 superpowers. Together, the new trio faces phenomenal challenges as they discover the dark secrets of Mako Island. They use their combined powers to protect the island, face off a catastrophic event and realize their destiny as mermaids.