The lifes of three girls living on the Australian Gold Coast are changed forever. When Cleo, Emma and Rikki stumble upon a pool inside the dreaded Mako Island, a full moon transforms them and the next day they start turning into super powered mermaids! With the help of friend Lewis, the girls manage to control their powers and hide their secrets. But it becomes difficult with maddening full moons, nosey scientists and local bad boy Zane going on a quest to find them!
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.
Cleo's sister Kim finds her diary and thinks that Cleo and her friends are 'evil mermaids' and she plans to tell everyone. Meanwhile, Lewis goes to Mako Island to find out more about what happened to the girls.
Emma gets into trouble babysitting her little brother Elliot. She goes to Lewis for help when Elliot develops a crush on Rikki.
Lewis is offered a great job with a marine biologist, but the girls are concerned that his interest will cause him to reveal too much information
Rikki is offered a job to collect rare fish. At first it seems perfectly fine, until she finds out the person that hired her is selling the fish for large profits.
Byron asks Emma for help with training the school's swimming team and Emma has some trouble deciding whether or not to help out in her current situation. Meanwhile, Lewis continues to experiment on the girls, which annoys Rikki.