Emma, Cleo and Rikki, three different teenage girls suddenly find they've got extraordinary power over water after a boat trip leaves them stranded on a mysterious island, but that's not all, they find that 10 seconds after they touch water, they grow tails! 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 too strong for Cleo and when she is nearly exposed, she must turn to Lewis for help. Also Rikki discovers her power.
Cleo does not 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 frenzy 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.
Cleo gets a job feeding the dolphins at a marine park and Emma plans a sleepover.
Emma decides that she is not going to have her annual sleepover party due to what has happened to them. But Rikki and Cleo try to convince her otherwise. Meanwhile, Lewis conducts experiments on the girls.
Elliot falls for Rikki when she saves him from drowning. But Rikki harshly turns him down and Elliot responds by running away. She finds him hiding in a plant nursery, where they both get locked in just as the sprinkler system is about to start up.
The girls ignore Miss Chatham's advice to beware of trouble when there's a full moon.
On the verge of discovering how the girls transform, Lewis considers revealing their secrets to a beautiful marine biologist.
While trying to buy a fish for Cleo, Rikki finds herself involved in a fish-smuggling operation.
The influence of the full moon causes the girls' powers to be enhanced and wreak havoc on Lewis in his boat.
Emma uses her new found powers to help Elliott win a soccer match. Meanwhile, Lewis decides to test the girls' increased powers.
Zane returns to town and tries to win back Rikki. Meanwhile, Cleo is becoming suspicious of Lewis, due to his spending more time with Charlotte.
When Rikki goes to stay with Emma and her family, she gets caught up in Emma's rebellious behavior, and takes part in a house party. But when Emma's parents come home earlier than expected, heads roll.
Lewis finds an old book on mermaids and tests out a potion recipe on the girls. Meanwhile, Cleo decides to use her powers to get Don and her mother back together.
Cleo is shocked when she discovers that Don is dating Charlotte's mother. Meanwhile, Lewis learns about Charlotte's feelings for him.
A dolphin goes missing from the Marine Park after a mermaid prank.
Rikki must overcome her shame about her background and introduce her father to Zane.
Emma meets a handsome horseback riding instructor, and uses her powers to heal an injured horse. Meanwhile, Lewis invites both Charlotte and Cleo to an awards dinner.