Cleo discovers a comet is causing fish to flee the waters around Mako Island. The girls find a way to restart the message in the Moon Pool and discover the comet will collide with earth. Sophie meets Ryan and with Zane's help they join forces to remove the crystals from the Moon Pool.

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.

Cleo seeks a new job as an assistant dolphin trainer at the Marine Park. Rikki fires Nate's band from playing at the cafe until she and Bella reach an understanding.

Sophie and Ryan damage the Moon Pool in their efforts to obtain the rare crystals. The girls improvise and use their own magic to prevent the impact of the comet and a planet wide catastrophe. Rikki, Cleo and Bella graduate with a surprise.

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.

Zane installs Sophie as head of staff at the cafe. Bella develops a sneezing allergy that also accidentally triggers her powers. Sophie seizes on the problem and looks to fire Bella and the band.

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.

The girls discover the Mako crystals contain immense energy and need to mount a rescue mission when Zane jealously steals one in an effort to gain Rikki‟s attention.

Cleo defies Lewis' advice and experiments with the magic Moon Pool water. Using her powers, Rikki accidentally gets the water into Cleo's aquarium and all the girls have to rescue Cleo's fish Hector.

Bella helps Will with his free-diving training but when she realises he's more interested in her as a mermaid than a girl, she's hurt. Rikki and Cleo have to improvise when the cafe's first corporate function is actually a children‟s party.