When three normal teenage girls stumble upon a ancient cave they undergo a transformation that will change their lives forever. This is H2O Just Add Water
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.
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. Meanwhile 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.
The arrival of Cleo's cousin Angela is seen by the girl's 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.
Emma invites Miss Chathams to stay at her place and Zane overhears Miss Chathams concerning about 'her treasure'.
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, Kim begins to suspect a ruse, but a more serious threat develops when a containment unit arrives to take the girls to a hospital for study.
When Zane causes the enigmatic Miss Chatham to be evicted from her boat, Emma invites her to stay at her home. Overhearing her concern for her "treasure", Zane chases after Miss Chatham seeking compensation for his damaged jet ski 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 being seen as a mermaid to save him.
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.
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
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.
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.
Emma gets into trouble babysitting her little brother Elliot. She goes to Lewis for help when Elliot develops a crush on Rikki.
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.