As a new student as soon as Charlotte meets Lewis she develops a crush on him.
As Lewis' new friendship with Charlotte grows to Cleo's dismay, so do Cleo's suspicions about her, but she insists it has nothing to do with jealousy. Cleo continuously embarrasses herself when she falsely accuses Charlotte of several things both privately and publicly, such as Charlotte having a boyfriend called RJ or Charlotte setting her old school's art room on fire.
Charlotte is in the background while Lewis and Nate fight. It seems she might be on Lewis' side.
Cleo becomes concerned with Lewis spending time with Charlotte. Lewis insists things between them aren't as they appear, but perhaps he needs to tell that to Charlotte, who is convinced that he's not over Cleo. 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 Cleo's troubles, she meets her daughter...Charlotte! When Lewis comes over to convince Cleo that there is nothing going on between Charlotte and him, Charlotte keeps Lewis to herself with the help of her controlling mother and purposely asks Cleo if spending time with Lewis would be a problem, this upsets Cleo so she and Emma and Rikki use their powers to ruin Charlotte and Lewis' dinner. The next day, Lewis tries to tell Charlotte that he is not interested in her, but she says that she can wait, causing Cleo to gain a new enemy.
Cleo is dealing with a problem concerning Charlotte and Lewis' jacket, Charlotte asks Cleo to return Lewis' jacket to purposely upset Cleo, Lewis tries to convince Charlotte that they are only friends but she insists that he is wrong and will do anything to make Cleo jealous. In the end, Zane fixes Rikki's father's bike and they become good friends. Because of this, Rikki gives up from embarrassing and invites Cleo, Emma and Lewis to her house for a party. While at the party, Lewis gives Cleo his jacket after telling her that he only gave it to Charlotte because she asked him to.
Lewis is nominated for a prize and invites Cleo, but she thinks it is not a good idea since that they are only friends now, but with Charlotte desperate for wanting to go out with him she invites herself along leading to Lewis to lie to both girls.
While Cleo is studying, Charlotte asks Lewis to bring her to Mako Island to sketch it. When Charlotte arrives to help Cleo, she gives Cleo more homework, so she can spend more time with Lewis. Lewis and Charlotte arrive at Mako Island, but they are followed by the girls. Charlotte tries to make a move on Lewis, but he tries to tell her that they are only friends, but is interrupted by Cleo falling into the water. Charlotte hears it, but Lewis spots Cleo and stops Charlotte from looking. Charlotte mistakes Lewis' actions and kisses him, which makes Cleo very angry. The next day, Cleo took her biology exam again and passed. At the JuiceNet Café, Charlotte "makes" Cleo thank her for her tutoring, and Lewis tries to tell Cleo the truth, but fed up with his relationship with Charlotte, Cleo tells Lewis that he can do what ever he wants.
At the marine park, a little girl sees Emma as a mermaid and she tells that to her mother, but no one believes the girl. Cleo, Rikki and Lewis try to get Emma back home. However, Lewis has a date with Charlotte at the same time. While looking for Lewis, Charlotte ends up bumping to Cleo and thinks that Cleo wants to ruin their date.
The revived Sertori family camping holiday is ill-fated not just by choice of location, Mako Island, but in the unfortunate timing of coinciding with the Full Moon. Cleo's friends and Charlotte tag along to help one another stay out of trouble but Lewis' plans are soon tested. Later that night, Lewis and Charlotte go for a romantic walk. Discovered Charlotte's disappearance, Kim opens the "moonlight proof" tent, causing Cleo to get affected by the moon. Kim finds Lewis and Charlotte who almost kiss. Lewis takes Kim back to the camp, leaving Charlotte alone. Emma and Rikki eventually discover Cleo is gone. Despite Lewis' protests, they leave the tent and face the powers of the moon. Charlotte bumps into Cleo, she torments Cleo because she thinks Cleo is jealous of her new relationship with Lewis. Angry, Rikki and Emma who are watching nearby strike Charlotte with lightning, scared Charlotte runs away and finds the Moon Pool but at the height of the full moon may have unforeseen consequences. The girls defeat the power of moon and Emma freezes the moon pool so Charlotte can't become a mermaid. Lewis arrives and brings Charlotte back to the camp. The next day, the girls and Lewis have to find out Charlotte is a mermaid or not. Lewis "accidentally" splashes her with a drink to find out, but nothing happens, so Charlotte isn't a mermaid.
After confiding in her that she misses spending time with Lewis, Cleo takes Rikki's advice to re-establish her friendship with him and to embrace her unique abilities; after all, Cleo is no ordinary girl, but Charlotte is. Cleo tries to spend as much time with Lewis as she can with out Charlotte in the picture to make him realize that she is the one for him, but Charlotte misunderstands this thinking that Cleo is just trying to make her jealous so she does the same.
Afraid his dream coming true, Lewis has to help her protect her secret; he does so by finding the orange bear Kim threw into the river and claiming that's what looked like Cleo's orange tail, later Cleo thanks him and they have a very awkward intimate moment which is disrupted by Charlotte.
Lewis and Charlotte decide to have a picnic. At the same time, Cleo tells Lewis about the anniversary and invites him to her house to celebrate. Lewis is forced to make terrible choice: Have a picnic with Charlotte or celebrate with Cleo. Lewis tells Charlotte he must help Cleo. Lewis tells Cleo he has a picnic with Charlotte. Lewis leaves. Lewis tells them he has a picnic with Charlotte. Charlotte is waiting for Lewis. Lewis finds Charlotte and they finally celebrate the picnic
When Cleo is out of earshot, Max tells Lewis that Cleo and Lewis should never have broken up because he "knows when people should be together". Lewis discovers a powerful ally in Max, who helps him continue on with his research. Charlotte is upset because Lewis now is avoiding her.
Charlotte calls, because she must meet up at the JuiceNet a hour ago. Lewis hurries there and Charlotte is angry for that Lewis is spending time with the girls. Lewis lies to her this is a project.
Lewis meets up with Charlotte and Charlotte says Gracie said the stories about the sea. Charlotte sees the locket and says the girls got the same locket.
At the Mako Island, the girls tell Lewis that Charlotte has taken the film. Back at the JuiceNet, Charlotte watches the film and sees the part of the film in which Max tossed Gracie to the moon pool and turned into a mermaid as Lewis arrives.
Moments later, Lewis doesn't believe Charlotte and lies to her that there's no such thing as mermaids. Back at Cleo's house, Lewis tells the girls what happened and the girls order him to stop Charlotte.
Charlotte appears and shows Lewis the pictures from the film. She shows the picture of Max and says she knows about him, because she found one of Gracie's sketches. Charlotte asks Lewis in help in looking for him, but Lewis disagrees. Charlotte sees the bag with the research inside Lewis' backpack.
Later on at Max's house, Lewis gives back the research and lets him to guard it, but he's unaware Charlotte has followed him. Moments later, Charlotte meets Max and wants to discover the all secret.
Back at Cleo's house, Lewis tells the girls Charlotte knows about Max. At the JuiceNet cafe, Charlotte yells at Lewis for lies and tells him Gracie was really a mermaid. She also says she was talking with Max, because she followed and saw the return of the research. Charlotte wants to know the truth, but Lewis can't tell and Charlotte runs away from him.
Lewis arrives and tells the girls Charlotte has followed him. Lewis says he was sure Max will never talk something to Charlotte. Emma asks Lewis he did talk with her. Lewis tells Emma he and Charlotte start an argument and she disappeared. Rikki decides to find Charlotte, but Lewis protests, because the moon will soon rise. Emma yells Charlotte knows about the secret. Rikki asks Lewis Charlotte is still unaware of them and Lewis says Max was talking about Gracie. At Cleo's house, while the moon rises, Lewis tells Cleo he and Charlotte almost broke up. Lewis says he didn't gave Charlotte the secrets, only the lies.
Lewis finds Charlotte standing and decides to break up with her. Lewis tells her he's sorry for the last day. Charlotte says she was jealous for the time in which Lewis was spending with the girls and was fighting with it. Lewis tells her they're friends and got the cases. Charlotte says these are secrets. Lewis is going to say abut breaking up with her, but Charlotte thinks Lewis must tell the truth, but now she knows the truth. Charlotte then says she wants to show him something. Charlotte jumps into water. She then shows up as a mermaid. Lewis becomes very surprised.
Lewis and Charlotte meet up at Mako Island and she shows him the powers of fire and controls it. Charlotte asks how much time the girls controlled their powers and Lewis says only a few weeks. Charlotte then says she's better. Charlotte tells the girls Lewis' birthday are coming. Cleo decides to have the party boat, but Charlotte refuses her idea and decides to make the surprise party, much to Rikki's chagrin.
Charlotte meets up with Anette and says Lewis' birthday is coming and so Charlotte decides to have a party at her house because Anette has the beautiful garden. Anette asks Charlotte to bring the chairs from the pool room. Charlotte brings the chairs, but sees the water is dripping from the broken pipe, so she freezes the water. Back at the cafe, Charlotte decides what food and music to have at the party than Lewis' favorite.
Meanwhile, Charlotte lies to Lewis she promised Anette to clean the pool and needed Lewis' help.
The next day at Charlotte's house, Charlotte has invited her friends from the old school. Cleo, Emma and Ash (who was also invited to the party) question Charlotte where Lewis' friends are and Charlotte explains they're Lewis' friends. Lewis appears and is surprised to see the party. Charlotte introduces her friends to Lewis.
Lewis gets bored and admits to Charlotte he hates all that Charlotte planned for the party. With that, everybody listen to music and see the party boat with Lewis' friends. Lewis asks Charlotte if this was also her idea and Rikki explains this was hers, Emma and Cleo's. As Lewis starts think that's why they disappeared, Charlotte is unhappy to see Rikki at the party and the girls say that from now Charlotte must work alone.
Charlotte's short-lived friendship with the girls is over. She demands loyalty from Lewis by saying he cannot talk or have anything to do with Rikki, Emma, or "especially Cleo". Lewis only reluctantly agrees, hoping it's just temporary. Cleo escapes and meets Charlotte. Using her unfair advantage - her additional powers and strength - Charlotte beats Cleo in a brief showdown, steals her locket, and tells Cleo that Lewis no longer wants anything to do with her. Rikki and Emma tell Cleo to stay away from Charlotte and Lewis, and no longer listen to her, refusing to help. Feeling crushed, heart-broken and helpless, she tries to call Lewis, but Charlotte has stolen his phone. Heart-broken, isolated and overpowered, Cleo tries to escape her human life, she runs away from home, only to find herself in a shark breeding ground. Lewis finds out that Charlotte stole his phone and is trying to isolate him. Angrily, Lewis breaks up with her, saves Cleo, and gets back together with her and they kiss. Unfortunately, by choosing Cleo over Charlotte, Lewis has created an enemy more powerful than they could imagine.