Three different teenage girls suddenly find they've got extraordinary power over water. Their lives will change forever as they learn more about their own potential and the power of friendship.
Despite knowing what water can do, the temptation of Miriam's pool party proves to strong for Cleo and when she is nearly exposed, she must turn to Lewis for help. Rikki discovers her power.
Catch of the Day
Cleo doesn't share her friends' enthusiasm for their new-found abilities. Rikki and Emma's efforts to rescue sea turtles trapped in fishing nets causes a media frenzie about sharks patrolling the nearby waters. Believing her father to be involved, Cleo takes it upon herself to go after the boat responsible for the illegal fishing practices but finds herself caught up in a net herself.
Emma travels back to Mako Island where she finds a mysterious necklace at the bottom of the moonpool. The risks associated with Cleo's new job at the marine park give her friends cause for concern but Cleo's determination to lead a normal life helps convince Emma to set aside her own concerns and go ahead with her annual sleepover party. Lewis experiments to determine what will change the girls.
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 the cause of the girls' transformation.
While babysitting Emma's brother Elliot, Rikki risks exposing her secret when he get's 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.
Cleo receives a cryptic warning from Miss Chatham but doesn't know quite what to make of it until Emma comes under the spell of the full moon during her father's birthday party and is unable to change back from a mermaid.
The Denman Affair
Rikki lands a seemingly harmless job collecting rare fish, but when she learns the man who hired her doesn't work for the marine park but is instead a dealer selling the fish for big profits, she decides to take on the task of setting things right.
The Camera Never Lies
A short film contest on the subject of heroes captures the focus of the girls' attention but a film about Emma's mother isn't Rikki's idea of fun and she decides to use her abilities to capture some impressive underwater footage. Inelligible to compete himself, Zane recruits Lewis to document a recreation of his father's famous windsurfing trip to Mako Island.
Sink or Swim
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", all the while insisting to Rikki and Cleo that her interest has nothing to do with spending time with Byron and everything to do with seeing Zane defeated. Lewis continues his experiments on the girls but Rikki doesn't like being treated like a specimen and does her best to disrupt his research.
The Siren Effect
After a humiliating public karaoke performance, Cleo's sudden newfound singing ability on the night of a full moon gives her friends cause for concern. When she develops a mysterious Siren-like hold over the opposite sex, a sudden rush of admirers gives Cleo the audience she's dreamed of while her panicked father struggles to control the growing crowd.
When Zane causes the enigmatic Miss Catham to be evicted from her boat, Emma invites to stay at her home. Overhearing her concern for her "treasure", Zane chases after Miss Catham seeking compensation for his damaged jetski 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 the water to save him.
Cleo is counting the days until her 16th birthday, fully aware that her big day can only be ruined by one thing — another of her father's embarrassing birthday parties. Zane embarks on a hunt for the "sea monster" he saw when he nearly drowned and recruits Lewis to help, but Lewis has promised the girls to do everything he can to keep Zane well away from them and Mako Island.
The Big Chill
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 her new boyfriend have her family and friends determined to discover the identity of Cleo's mysterious boyfriend. Events come to a head when Kim overhears Cleo on the phone and comes to the conclusion that she's run away with Zane to elope. Meanwhile, Zane uses Cleo to gain access to the marine park's library to continue his sea monster research.
Under the Weather
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, Cleo's sister Kim begins to suspect a ruse but a more serious threat develops when a containment unit arrives to take the girls to hospital for study forcing Emma to admit everything.
Bad Moon Rising
Haunted by his encounter with the "sea monster", Zane steps up his investigation which leads him to Mako Island. Lewis' efforts to help the girls prepare for the night's full moon go for naught as Rikki catches a glimpses of the full moon causing her to lose control of her powers. Emma, Cleo and Lewis follow her to Mako Island where Zane has already found a distraught Rikki.
The arrival of Cleo and Kim's cousin Angela is seen by the girls' father as an opportunity for Cleo to learn some maturity and responsibility. However, chaperoning a headstrong girl with a fondness for water quickly brings on its challenges, and with a birdnapped pelican added to the mix, makes for an eventfull afternoon. At the end of the day however, Cleo and her sister to begin appreciate one another... if only temporarily.
Hook, Line and Sinker
Lewis' excitement about testing his new lure in the annual fishing derby is quelled by Cleo's refusal to renew their traditional partnership for fear of getting wet. When Lewis struggles, Cleo decides to use her mermaid powers to help Lewis land "the big one". However, when Lewis is accussed of cheating, he is kicked out of the competition by Cleo's father and forbidden from ever seeing her again. Rikki and Zane encounter one another at a business seminar and get trapped on a balcony. The time alone together allows them to get to know one another better, perhaps a little too well when the rain starts to fall.
An innocent comment about her dependable nature convinces Emma she needs a change — and what better way to start than to colour her hair! Zane's "sea monster" investigation leads him to Miss Chatham who he is convinced is at the centre of the mystery. Emma's attempt to protect Miss Chatham's secret provides Zane with his first clear look at a mermaid. Rikki agrees to go out with Zane on the condition he stop his talk about mermaids. It is an easy deal for Zane to make.
Fish Out of Water
Emma and Cleo find out Zane and Rikki are an item and force her to choose – him or them. When Zane appears to side with Harrison over the Mako island development Rikki’s decision is clear. Now it’s Zane’s turn to make a choice…
In Too Deep
Rikki's date with Zane becomes a subject of speculation for Emma and Cleo. Concerned about Zane's trustfulness, Emma gives Rikki an ultimatum: her and Cleo or Zane. It's a choice Rikki resents being asked to make and it quickly drives a wedge between the girls. Harrison Bennett's unveiling of plans to develop Mako Island realigns feelings as Emma joins Rikki's vocal opposition and Zane takes a stand against his father. Cleo attempts to liberate herself by thinning her wardrobe.
Love Potion #9
With the school dance approaching, Emma and Cleo are fretting about having to ask Byron and Lewis to go with them, relying on the boys to make the first move. But Byron doesn't ask Emma, who he thinks is too busy heading up the organising committee, and Lewis is too caught up in his work on a new scientific way to make it easier for the girls to be mermaids by making them waterproof. Only when Cleo leaves for the dance with Nate does Lewis realise his mistake. Has Lewis missed his chance with Cleo and might his untested waterproofing spray ruin the evening for everyone?
The prospect of going on vacation to a resort with her family doesn't sit well with Emma and her excuses don't go over well with her parents or brother who are all looking forward to their annual trip. The return of marine biologist Dr Denman [episode 8] concerns Lewis and the girls but Zane sees a possible ally (or worse, a competitor) in the search for his mermaid. With Mako Island firmly in Denman's sights, the girls' secret is on the verge of being blown out of the water.
A Twist in the Tail
The hunt for the now identified mermaids intensifies as Dr Denman sets a trap for the girls at Mako Island forcing Zane to make a decision about where his loyalties lie. A lunar eclipse gives the girls an opportunity to free themselves from further pursuit by sacrificing their powers.
The season 1 was filmed in 2005 and 2006.