Cleo and Rikki are about to face their last 년 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 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 a new ally in Bella, the 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, form new relationships and realize their destiny as mermaids.