Cleo and Rikki are about to face their final year at school together without Emma, who’s gone overseas. They're confident they've overcome the affect of the full moon and know everything about Mako Island and its magic. Zane surprises Rikki with an offer for her to join him as a partner in a new upmarket cafe, while Lewis is taking his relationship with Cleo a little too comfortably. When Will, a boy with remarkable skills underwater stumbles upon the Moon Pool, he encounters a strange water force and is knocked unconscious. That night, at the opening of the new cafe, Rikki and Cleo meet Bella, a new girl who helps Rikki out of a tricky situation by stepping in to sing in Nate’s band. In a break outside, Rikki's attacked by water and dragged out to sea. Bella sees this and joins Cleo to rescue Rikki. Cleo's shocked to find Bella's a mermaid too! Rikki's saved from the encounter but they find the new boy Will in the Moon Pool. The girls realise something has changed on Mako Island, and it appears Will is now on the trail of their magic and secret.