Three teenage girls discover an underwater cavern on Mako Island and undergo a magical transformation.
The girls are trying to keep their fish tails a secret, but when socialite Miriam throws a pool party, all they can do is try to stay dry.
When the girls learn that sea turtles are getting caught in nets, it's Rikki and Emma to the rescue. But fishermen think they've got a shark problem.
While deciding to throw her annual sleepover party, Emma heads to Mako Island in search of a clue.
When Cleo's little sister gets ahold of Cleo's diary, she threatens to expose the three friends' secret.
Emma's younger brother, Elliot, develops a crush when he's saved from drowning.
A mysterious woman warns Cleo to steer clear of water and avoid looking at the full moon.
When Lewis takes a job as a research assistant to a noted marine biologist, the girls are worried that he might accidentally give away their secret.
When Rikki tries to catch a fish for Cleo, she takes on a job offer from a breeder to collect rare fish that isn't what it seems.
The girls participate in a film contest but the heroic theme leaves the group at odds.
A local surfing star asks Emma to be his swim coach but her secret makes her hesitant to dive into the role.
As a full moon approaches, Cleo flexes her new pipes and sings a siren song that has the boys all over town swooning.
Miss Chatham's mission to retrieve a precious piece of treasure lands her in a health emergency.
Cleo faces yet another birthday party thrown by her dad. Zane is out to learn the identity of the sea creature he thinks saved his life.
When Emma is put in charge of the cafรฉ for a weekend, her new employee Rikki has a tough time adjusting to her managerial style.
Zane continues the search for his sea savior. Meanwhile, Cleo's family and friends are convinced she has a secret boyfriend.
On the day of a big biology exam, a thunderstorm has the girls faking sick to skip school.
Zane's investigation leads him to Mako Island, where he runs into a familiar face.
When Cleo's cousin Angela comes to visit, she proves to be a wild child thirsty for fun in the water.
When Lewis enters a fishing contest, Cleo's plan to help him win backfires.
Emma takes on a bold new hair color. Zane's sea monster mystery leads him to Miss Chatham and her old homestead.
When Cleo and Emma find out about Rikki's new romance, Rikki must make a tough choice.
When Rikki spies a locket with an uncanny resemblance to Cleo's in a jewelry shop, Zane doesn't understand her desperate desire to possess it.
Emma and Cleo hope that Byron and Lewis will ask them to the school dance. Meanwhile, Lewis comes up with a special formula for the girls.
Emma drops out of her family's annual vacation. Dr. Denman makes a discovery that brings her closer to the girls' secret.
Cleo, Emma and Rikki must evade a trap set by Dr. Denman.