When the full moon affects the girls in unexpected ways, they lose the ability to control their powers. Lewis finds himself on the wrong side of the girl's new abilities. Cleo is upset being made to accompany Kim to a concert.
Fire and Ice
Lewis has little success when he decides to test the girls' new and unpredictable powers. Emma helps her brother Elliot deal with a meddlesome teammate. Lewis' constant attention is driving Cleo crazy, leading to a frank discussion about their relationship.
Zane's reappearance forces Rikki to decide if staying away from him is what she really wants. Can Rikki become involved with Zane again and still keep her secret? As Lewis' new friendship with Charlotte grows, so do Cleo's suspicions about her, but she insists it has nothing to do with jealousy.
With Emma's parents away for the weekend, Rikki comes over to stay but her behavior rubs off on Emma who decides to prove that she too can be a rebel. But is she trying too hard? And can she handle the consequences that come with throwing a party? Lewis' tries to become more manly when Nate tells him it may have been why Cleo broke up with him. Cleo gets a secret admirer.
When Lewis presents the girls with a rare book he discovered about mermaid mythology, Rikki and Emma reluctantly agree to help Lewis and Cleo make a potion that will give the girls the power to grant wishes. For Cleo it's an opportunity to reunite her fractured family, but when the project grows out of control, they must act quickly to clean up the café before Wilfred arrives to shoot a commercial.
Cleo's life is in turmoil as she is seeming confronted with Charlotte and Lewis at every turn. Lewis insists things between them aren't as they appear, but perhaps he needs to tell that to Charlotte. To make matters worse, Cleo's dad has invited a woman over for dinner. And just when it appears that welcoming a potential stepmother is the worst of all Cleo's troubles, she's introduced to the woman's daughter…
Hot Water In
In need of money for a new computer, Lewis decides it is time he finds a job, and as luck would have it, a position becomes available at the aquarium when Cleo is fired. In an effort to help get Cleo her job back, Rikki and Emma set out to prove that serving ice-cream can be the hardest job in the world. But when the the aquarium's star attraction goes missing, Lewis becomes the prime suspect.
Wrong Side of the Tracks
Zane and Rikki's rocky relatationship is once again put to the test when her efforts to hide her personal life from those around her collides with Zane's determination to involve himself in her life.
Riding for a Fall
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 is failing at school and is assigned a tutor - Charlotte. Cleo wants Lewis back but her attitude to Charlotte only drives him further away. Cleo discovers Lewis and Charlotte kissing.
Zane hears of a treasure under the ocean and enlists the girls to find it. Rikki is injured in the search. Zane gives Rikki the reward. Charlotte contrives to keep Lewis from the girls.
Monster In The Water
Emma starts acting weird after pricking her finger on a barb of coral. Cleo's fish Hector exhibits the same behavior. Emma becomes more and more like a fish and Lewis has to find a way to cure her.
Camping at Mako Island
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 realizes something weird is going on there.
Cleo vs. Charlotte
Emma thinks she's going to get a promotion at the juice bar but Ash is given the job. Emma and Ash clash over the job, but we discover it's over a lot more. Meanwhile, Lewis stands up to Charlotte over spending time with Cleo.
Lewis and Zane find an old myth about mermaids is true - the scent of ambergris sends them wild. But it backfires when sleazy Nate gets hold of the scent and has all three mermaid girls on his arm!
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.
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.
The Heat is On
When Zane and Ash clash at the juice bar, Rikki and Emma take their boyfriend's sides. Cleo is left alone celebrating their one year anniversary of becoming a mermaid - until Lewis arrives just in time to help.
The Gracie Code (Part One)
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.
The Gracie Code (Part Two)
Ash comes close to finding out about Emma. Meanwhile, the girls discover that Charlotte's grandmother was Gracie. Old wounds open up for Charlotte and she finds part of Max's research: an old movie of Gracie - as a mermaid!
And Then There Were Four
Charlotte follows Lewis to Max Hamilton. Max tells all to Charlotte when he discovers she's the granddaughter of his one true love. Charlotte and Lewis almost break up until she goes to the moon pool, becoming a mermaid herself!
Bubble, Bubble, Toil and Trouble
The girls have to deal with a fourth mermaid who turns out to be a loose cannon; Charlotte almost turns into a mermaid at a volleyball competition. Charlotte discovers she has all three mermaid superpowers.
As the girls continue to have problems accommodating Charlotte as a mermaid, Nate bullies Lewis. Despite the girls' attempts to keep Charlotte under control, she uses her powers on Nate and almost exposes them all.
Charlotte takes over and organizes a surprise party for Lewis, annoying Rikki. At the party, Charlotte is jealous of Cleo and traps her and Emma in a pool room. Rikki comes back just in time. From now on, Charlotte is on her own.
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.
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.
The season 2 was filmed in 2007 and 2008.