Cleo and Rikki are about to face their last tahun 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.
