The girls reluctantly agree to let Lewis get a research assistant's position with a marine biologist hoping to determine what's behind their transformation, but the girls have cause for concern when his enthusiasm brings him close to revealing too much information. After leaving behind a DNA sample from Cleo, Lewis is soon offered a lucrative job and has a choice to make about his future.
Byron asks Emma for training help prior to the school swimming meet. After initially turning him down, she changes her mind but becomes a "heavy training dragon". Lewis continues his experiments on the girls but Rikki does not like being treated like a specimen and does her best to disrupt his research.
After a humiliating public karaoke performance, Cleo's sudden new-found singing ability on the night of a full moon gives her friends cause for concern. When she develops a mysterious Siren-like hold over boys, a sudden rush of admirers gives Cleo the audience she's dreamed of while her panicked father struggles to control the growing crowd.
While babysitting Emma's brother Elliot, Rikki risks exposing her secret when he gets into trouble surfing. The girls turn to Lewis for advice when Elliot develops a crush on Rikki but the abrasive Rikki has a hard time letting him down gently, resulting in him running away. When Rikki finds him to apologise, however, she risks revealing the mermaid secret to him.
When Zane causes the enigmatic Miss Chatham to be evicted from her boat, Emma invites her to stay at her home. Overhearing her concern for her "treasure", Zane chases after Miss Chatham seeking compensation for his damaged jet ski when she departs the marina in her boat. Emma and Lewis arrive in time to help with a medical emergency but when Zane gets trapped on the sinking boat, Emma must risk being seen as a mermaid to save him.
Cleo's secret diary falls into her sister's hands and Kim's suspicions quickly grow into a plot to expose Cleo and rest of the "evil mermaids" during the Miss Sea Queen pageant. Lewis travels to Mako Island to try to find out the cause of the girls' transformation.
A sudden thunderstorm forces the girls to feign illness to stay home from school, but in faking their symptoms too well, they attract the attention of doctors eager to study their "rare tropical disease". As the number of people in quarantine grows, Kim begins to suspect a ruse, but a more serious threat develops when a containment unit arrives to take the girls to a hospital for study.
Despite knowing what water can do, the temptation of Miriam's pool party proves too strong for Cleo and when she is nearly exposed, she must turn to Lewis for help. Also Rikki discovers her power.
Emma is excelling at her new job at the JuiceNet café, but when Wilfred leaves her in charge for an entire weekend, her no-nonsense managerial style puts her at odds with her newest employee: Rikki. Miriam becomes the victim of her own scheming when Emma accidentally freezes her in the café's cool room forcing the girls to put aside their differences to defrost Miriam.
Cleo attracts the attention of a new dolphin at the marine park, but rumours about a new boyfriend have her family and friends determined to discover the identity of Cleo's mysterious beau. Meanwhile, Zane uses Cleo to gain access to the marine park's library to continue his sea monster research.
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