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.
Emma meets Ash - a handsome horse riding instructor. A horse falls ill and Emma must use her powers to fix it. Lewis accidentally invites both Charlotte and Cleo to an awards dinner.
Cleo and Rikki push Ash into asking Emma out again – little did they know he'd turn up at Emma's place in time for a full moon! Rikki and Cleo are both moonstruck and Ash looks set to discover Emma's secret.
Bella attempts to start a relationship with Will until she sees he's apparently got another girl. Lewis tests the magic Moon Pool water at school and attracts the attention of their Science teacher.
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.
Lewis has a bad dream about mermaids being discovered. It threatens to come true when Elliot asks Kim out on a date to the marine park and Cleo finds herself in trouble underwater in front of a crowd of people.
Bella's eager to ask Will to the end of year party until she sees him asking Rikki. A big misunderstanding occurs which prompts Will to express his true feelings for Bella. Sophie tries to get Zane to change the name of the cafe.
Cleo is bullied by Charlotte and runs away to the ocean. With Cleo missing, everyone becomes more and more worried. Lewis discovers what Charlotte has been up to and breaks up with her. He finds Cleo and they kiss.
Don takes the family camping on Mako Island during a Full Moon. The girls are affected and need the moon pool for sanctuary. Charlotte almost slips in the moon pool and realises something weird is going on there.
Rikki, Emma and Cleo discover that sea turtles are being captured by fishing nets. When Rikki and Emma attempt to rescue the turtles by cutting the nets, it causes the fisherman to think that there is a rogue shark roaming their ocean.
Everyone is shocked when Cleo is fired at the Ice Cream Stand, and even more so when Lewis takes her place! Rikki and Emma strike back at him but end up going too far.
Lewis discovers someone called Max Hamilton did research fifty years ago on the strange properties of Mako Island. Lewis tracks the old man down and finds that he knows a lot more about mermaids than Lewis first thought.
Emma starts acting weird after pricking her finger on a barb of coral. Cleo's fish Hector exhibits the same behaviour. Emma becomes more and more like a fish and Lewis has to find a way to cure her.
The full moon rises, and with the planets lining up, it can remove mermaid powers for good. Knowing this, Lewis lures Charlotte to the moon pool but it backfires and Emma, Cleo and Rikki all look set to lose their powers.
Byron asks Emma for help with training the school's swimming team and Emma has some trouble deciding whether or not to help out in her current situation. Meanwhile, Lewis continues to experiment on the girls, which annoys Rikki.
The full moon has come again, and the girls are taking every precaution they can imagine this time. It proves to be in vain, however, when Rikki falls under the spell.
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.
The girls discover a necklace for sale identical to Miss Chatham's. Miriam causes a stir by buying it – then throwing it in the water. Rikki faces a choice whether to take the necklace and reveal her mermaidness to Zane.
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.