Cleo and Rikki face their last year of high school without Emma, who's left town with her family, but are confident they know everything about their mermaid powers and Mako's secrets. But when Will a freakishly skilled free-diver uncovers a new cavern on Mako, he unwittingly unleashes a force that turns water against them. Luckily for them, they befriend a new mermaid, Bella, who brings an all new energy to the group. It seems they may need her help as Zane, heartbroken at being dumped by Rikki, is out to exploit Mako's secrets and may destroy everything if the girls can't stop him.
Note: filming on series 3 will finish at the end of April 2009. The series will not air in Australia until late 2009.