Kim reads Cleo's Diary and finds evidence of her being a mermaid. She spies on the girls and decides Miriam is the head mermaid. Miriam and Cleo enter the Queen of the Sea contest where Kim tries to prove her theory but only succeeds in upsetting Miriam.
Emma comes by boat to Mako Island in the company of Lewis, seeking answers to how she, Cleo and Rikki became mermaids. Emma leads him to a cave, in which Lewis accidentally falls, and decides to get to the moon lake by the water. On-site Lewis begins research.
At JuiceNet Café, Cleo and Rikki talk about the Miss Ocean competition. After a while, Miriam and Tiffany approach them, informing them that they intend to take part in it, and offend the girls. During the argument, Cleo almost reveals the secret to the mermaid, but Rikki calms her down in time.
At Sertori's house, Bev dusts the living room when Kim appears, asking for Cleo's whereabouts. Bev does not give the girl a satisfactory answer, so she goes to her room herself to borrow a belt. She finds a strange diary written in cipher in her older sister's things. There are mermaid drawings everywhere.
When Cleo works in a marine park, Miriam and Tiffany approach her. Miss Kent buys ice cream from her, while bragging about her preparations for the competition. When Mrs. Geddes appears, Cleo informs her that she intends to take part in the Miss Ocean competition herself.
Kim decides to ask her mother about Cleo, conducting the "investigation" of the girl's diary, when her sister comes home. Cleo asks Bev to teach her how to sew to prepare for a competition. In his room, Sertori washes the aquarium, using power to keep the fish away from their cottage. Kim enters the room and asks her sister where the fish are. The girl answers that they are in the bathtub. After not finding anything in the bathtub, Kim returns to his sister's room, where he notices animals in the aquarium. Cleo says the fish were in a bucket. Kim is becoming more and more suspicious.
Sometime later, Kim reads the diary, hiding it behind a book. When Bev accidentally spills Cleo with water, she runs into the bathroom, and her younger sister follows her. She sees the tail, but then the door closes.
Terrified, Kim goes to the Gilbert house to warn the mermaids of Emma's brother Elliot. She shows him the diary and tells him her assumptions. The boy doesn't believe her until, when in the evening - when he is taking a shower - he is "attacked" by ice cubes accidentally created by Emma.
The next day Elliot meets Kim. He tells her what happened last night. In his home, the boy tries to splash Emma with water, but he fails. He tells his sister that they shouldn't have secrets for themselves as siblings, then asks why she didn't tell him about being a mermaid. Emma pretends she doesn't know anything.
Emma goes to Cleo's house to talk to her about the whole incident. From her he learns about the diary and they both start arguing. Cleo thinks nothing happened until he suddenly notices the disappearance of the diary. Later, she finds it under Kim's pillow.
Cleo, Emma and Rikki are sitting in the JuiceNet Café when Kim and Elliot come to the cafe. With Lewis's help, they get on the set to fool their younger siblings Cleo and Emma - they want them to believe that Miriam and Tiffany are mermaids. Meanwhile, young Sertori also includes Miriam, Tiffany and Lewis among those suspected of being merpeople. Later, Kim and Elliot follow Mrs. Miriam Kent and her friend, they see strange waters in their presence.
Kim and Elliot devise a plan on how to unmask mermaids. During the competition, Elliot and Kim splash Cleo and the other girls with water. Cleo uses her powers not to get herself wet. Wet and furious, Miriam tries to hunt down Kim for destroying her chance to win. Tiffany wins the competition.
Cleo goes to Kim and lies that the diary was homework and the tail in the bathroom was the first version of the costume. Cleo sinks the diary into a lake on the island of Mako.
- First appearance of Elliot Gilbert.
- Lewis goes into the Moon Pool for the first time.
- The coaster is the Lethal Weapon – The Ride at Warner Bros. Movie World in Oxenford, Australia.
- Lethal Weapon – The Ride is now known as Arkham Asylum – Shock Therapy.
- Rikki boiled the water right underneath Miriam's feet, so she should've noticed.
- Is it really safe to have a roller coaster above the stage?
- Cleo is the only one who didn't get wet in the stage, but why didn't the audience notice?
- When Cleo suspends the fish, Kim should have been able to notice it, or at least notice Cleo glancing upwards.
- Ariu Lang Sio as Wilfred is credited, but do not show up in the episode.
- A book based on this episode is H2O: Sequins for Sea Queens.
- International Airdates:
- Denmark: February 9, 2008 on Jetix
- Poland: September 14, 2007 on Jetix
- This episode aired in the United States on March 30, 2008 on Nickelodeon.
- Lewis: So, this is the place that turned you into a fish.
- Emma: A mermaid, Lewis.
- Lewis: Yeah, that's what I meant. I don't know, I expected something a little more Club Med.
- Lewis: So, how did you get here?
- Emma: Swim. There’s an underwater entrance.
- Lewis: Swim? So the hike was optional. I could have swum?
- Emma: Fifty meters, without coming up for air?
- Lewis: Right, point taken. Air is good.
- Cleo: I'm a Portuguese Man-Of-War. (audience doesn't react) It's a jellyfish. (everyone claps)
- Emma: Rikki, at least try and pretend you're having fun.
- Rikki: But I'm not.
- Kim: Of course, Miriam's their leader.
- Elliot: Are you sure?
- Kim: Miriam's always the leader.
- Emma: I can't go near the river as...
- Lewis: Yeah, I know, the whole beached whale thing.
- Emma: Mermaid, Lewis.
- Lewis: Yes, that's what I meant.
- Kim: Lewis is one of them too.
- Elliot: I can't see Lewis... as a mermaid.
- Kim: No you dork, he'd be a merman.
- (After listening to Kim talk about how their sisters and Rikki are mermaids)
- Elliot: I think you should leave now
- Kim: You have GOT to believe me.
- Elliot: See, that's the thing, I don't.
- Rikki Chadwick - Cariba Heine
- Emma Gilbert - Claire Holt
- Cleo Sertori - Phoebe Tonkin
- Lewis McCartney - Angus McLaren
- Miriam Kent - Annabelle Stephenson
- Tiffany - Alice Hunter
- Kim Sertori - Cleo Massey
- Don Sertori - Alan David Lee
- Bev Sertori - Deborah Coulls
- Elliot Gilbert - Trent Sullivan
- Neil Gilbert - Jared Robinsen
- Lisa Gilbert - Caroline Kennison
- Mrs. Geddes - Andrea Moor
- Libby McIntyre - Jenna Hudson
- Lifeguard - Liam Pierce