Cleo and Rikki face their final year of high school without Emma. They make a new friend in Bella and meet new boy Will – and encounter a strange new force on Mako Island that turns water against them.
Rikki receives counterfeit money at the cafe and is held captive by criminals on a boat. Bella and Cleo are forced to reveal the trio's secret to Will so they can rescue Rikki together.
Cleo's forced to put Lewis' plans for romance on hold to restrain Kim's out of control love life. During this Don meets Samantha, a National Parks Manager and they start to date.
Zane concocts a dirt bike race to publicise the cafe. When Rikki discovers it's a scam she and Bella decide to teach him and Nate a lesson. Cleo and Lewis make a big discovery about mixing the waters in the Moon Pool.
Bella's warned to back off from Will as he's on the trail of the girls' mermaid secrets. She gets too close to him and he finally discovers she's a mermaid.
