Avery's mom is a marine biologist who just got a job at the Australian Institute of Marine Life in Gold Coast, Australia. This means that they have to move...again. This angers Avery to a boiling point. So the day that Avery, her mom, and her twin brother Spencer finally arrive at their new house, Avery takes her and her brother's new speed boat out and strands herself on Mako Island. Minutes after she reaches the small beach on the island, it starts to storm really bad. So Avery decides to head inland in search of shelter from the raging storm. Where she finds and falls into a strange cave among the rocks. In the cave she finds the moon pool and after being startled by a loud noise, accidentally backs up one step too far and falls into the pool right as the moon centers itself in the volcano's cone. When the water started to bubble and glow Avery is frightened and immediately gets out of the pool and edges to the back left corner on the cave where she finds a small black bag with something hard inside it. Avery then stuffs the bag in her pocket and quickly scrambles out of the cave and back to her boat where she curls up and falls asleep. The next morning, Avery woke up to the sound of three different female voices. Not knowing who the voices belonged to and realizing that she was still on the island, Avery jerked awake and tried to sit up but instead flipped out of the boat and into the sand. When she looks up after falling face-first into the sand, she sees three girls about the same age as her staring at her in disbelief. When Avery notices that they are standing a little farther behind her, she turns over and discovers that she has a mermaid tail! The three girls, who are Emma, Cleo, and Rikki; explain that they are mermaids too and that they want to help her. But Avery insists that she doesn't need their help. When Avery finally gets home, she finds out that the bag that she had found in the cave contains a locket just like Emma, Cleo, and Rikki's along with a small note. The note explains that the locket belonged to Gracie's little sister Ellie, who had followed her sister to Mako one full moon(a few months after Gracie, Julia, and Louise became mermaids) and Ellie became a mermaid also. When Gracie found out, she had Max make a fourth locket with a green gem and FOUR waves on the back instead of three. About four months after Ellie had became a mermaid, she died(she was thirteen). So after Ellie died, Gracie put her little sister's locket in a black jewelry bag with a note and set it in the back left corner of the moon pool as a memorial for Ellie. After that, Gracie asked that none of them talk about Ellie because it was too painful for them to talk about. So that is why Miss Chatham or Max never said anything about Ellie to Emma, Cleo, Rikki, or Lewis. But when they show Miss Chatham Eliie's locket that Avery found, she tells them all about Ellie. This seems to help Emma, Cleo, and RIkki accept Avery as mermaid more and the foursome is created when Avery agrees to join the group.