Zane learns of the myth of Ambergris, a fragrance that can make mermaids fall for any man that wears it. When he buys it, Nate ends up wearing it and Emma, Cleo and Rikki all fall for him.
Lewis keeps dreaming that Cleo is discovered to be a mermaid. Elliot has a crush on Kim and she agrees to go on a date with him to the marine park. Don makes Cleo chaperone. Emma is suspicious of Rikki and Ash's private conversations.
The girls prepare for the full moon again and insist they don't need Lewis' help. Rikki and Cleo suggest Ash to be more bold with Emma. Following their advice, Ash decides to do a surprise visit which could uncover their secret.
Cleo plans a party at the moon pool to celebrate the one year anniversary of the girls becoming mermaids, but when Zane and Ash have an argument at the juice bar, Rikki and Emma take sides against each other and forget about the party.
Lewis is trying to find more information on the moon pool and stumbles across research on it by Max Hamilton from fifty years ago. Will Lewis reveal the girls' secret to him in order to complete the puzzle?
Charlotte's stubborn nature and thirst for knowledge leads her to Max, who confirms her suspicions and leads her to the Moon Pool. Lewis meanwhile is having trouble trying to keep their relationship together.
The girls decide that they should accept Charlotte, and the four work together when they need to join the volleyball team to avoid failing Physical Education.
Charlotte is determined to prove her love for Lewis and show she is better than the other three, and refuses their advice and takes matters into her own hands when Nate causes trouble for Lewis.
More friction arises, this time over Lewis' birthday party. Charlotte takes control and drives Rikki away from the party. However, things go out of hand and it is up to her to save the day.
Charlotte's short-lived friendship with the other girls is over and she demands that out of loyalty to her, Lewis have no further contact with Rikki, Emma and most of all, Cleo.
Lewis find out that there is a way for a mermaid to lose her powers and attempts to lure Charlotte to the Moon Pool. The plan backfire and it looks like all the girls may lose their powers.