Three teenage girls discover a secret pool when they are stranded on Mako Island.

Cleo, Emma and Rikki fear that their secret will get out when they attend a pool party.

Cleo must overcome her fear of water in order to enjoy being a mermaid.

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.

Elliot falls in love with Rikki after she saves him from drowning at the beach.