The Siren Effect


Air Date:
22 September 2006
Directed by:
Colin Budds
Written by:
Susan MacGillicuddy
Preceded By:
Followed By:

The Siren Effect is the 12th episode in Season 1 of H2O: Just Add Water.


The full moon strikes Cleo, turning her into a magical siren, attracting all the young males in the vicinity. Cleo enjoys the attention and is encouraged to sing for them, but a concert at the JuiceNet Café as the moon sets reveals her truly awful voice.


Another full moon approaches. Before nightfall, Rikki enters Cleo up for a karaoke contest, not realizing that she has no ability to sing and Cleo embarrasses herself in front of everybody listening. That night, Lewis and the mermaids sleep at Sertori's House to try avoiding the full moon. Cleo's protective father is suspicious of Lewis' wanting to stay over. While watching an old cheesy horror movie, Cleo catches a glimpse of the moon in a glass of water and starts to reach out for it, but Lewis grabs her wrist and she snaps out of it. She says she has to go to the bathroom and Lewis insists someone go with her, but she says she will be fine.

While in the bathroom Cleo sees the full moon in a reflection in the sink, and starts moving her hand through the water in the sink. Rikki had brought over a karaoke CD and using a mic set up to the speakers and TV, she sings rather badly. Before Emma is able to grab the mic, Cleo grabs it from Rikki and she is suddenly able to sing better than any other girl in town. She also becomes slightly self absorbed and extremely bubbly. Lewis becomes enchanted by the music, and so do other boys across the beach, including Byron. Emma and Rikki try to convince Cleo that this is not who she is, but she is as moonstruck as the boys are to her singing. Eventually, using headphones to pull himself out of the trance and drown out Cleo's enchanting singing, Lewis soon realizes she's become a siren, one of those ancient, mythological mermaids whose voices lured sailors to their deaths, but with Cleo's case while her singing is hypnotic to the boys, It just makes them fall head-over-heels in love with Cleo. When Rikki and Emma leave, Cleo starts walking towards Lewis (who is sitting on her bed) and Lewis, thinking she wants to say something to him, takes off his headphones, and he becomes so enchanted that both he and Cleo lean in and kiss.

Cleo's dad sees this and pulls out Lewis by force, much to his obvious dismay. Emma and Rikki force Cleo to go to bed and even sleep in her room on the floor to try and stop her, bit Cleo simply steps over them and heads downstairs to call a radio company so she can sing for them. All of the town's boys hear her singing over the radio, which causes them to fall into a hypnotic love trance for Cleo. In the morning, hundreds of boys gather on the Sertoris' lawn to hear Cleo sing. Cleo, to her father's irritation, escapes to the JuiceNet Café and sings in front of all the boys; the full moon sinks, causing Cleo's wonderful voice to go with it, thus ending the love trance with her, along with her brief burst of popularity, much to her and all the boys' obvious dismay. She is sad that the boys no longer like her, but her dad reassures her and says that he still loves her, and that he always has and always will. In the end, Lewis comes to Cleo's house both say that they are only friends, but they have strange feelings. Lewis' reaction of a smile after Cleo closes the door shows that although he was under a spell the night before, he does love Cleo.


  • Second full moon episode.
  • First time Cleo is affected by the full moon.
  • When Cleo is affected by the full moon in this episode she can be wet and not turn into a mermaid. She also has the powers of a siren, the ability to enchant men (well, boys her age) with her singing, and she also becomes extremely bubbly and kind of self absorbed about how everyone loves her and her singing, but more importantly her.
  • First time Cleo and Lewis kiss.
  • Only time Zane and Byron like Cleo.
  • This is the only episode so far we don't see any of the girls in mermaid form. Despite Cleo touching water she doesn't transform into a mermaid.
  • In ancient times, Sirens have been said to be bird-women, with feathers and a ability to fly.
  • Unlike Emma in Moon Spell, Cleo could remember the events of the past night.
  • The moon rises in the east and sets in the west: opposite sides of the sky. But in this episode, the moon is always seen right next to Mako Island. (One might try and explain this by saying that the camera is looking at different sides of the island. However, if you compare the shots, you can easily see that the silhouette is identical each time, proving that the camera is looking at the same side in each).
  • Songs Featured: "Steal the Secret", "Head Over Heels", and "Angels to Your Side"
  • The actor Dominic Deutscher from H2O's spin-off series Mako Mermaids plays one of the boys smitten by Cleo's singing.


  • In this episode Cleo becomes a Siren. In mythology, Sirens would sing to attract Sailors and make them fall to their deaths. However, when Cleo sings she only makes the boys become infatuated with her.
  • Rikki's calling Lewis a pod person is an obvious reference to the decades-spanning franchise.
  • Cleo brushing her pony tail so much whilst singing is possibly a reference to how in old myths mermaids were said to sit on rocks in the ocean and sing to passing ships as they brushed their long hair.


  • International Airdates:
    • Denmark: March 1, 2008 on Jetix
    • Poland: September 22, 2007 on Jetix
  • This episode aired in the United States on May 18, 2008 on Nickelodeon.


Emma: Don't look at the moon light!

Lewis (after grabbing a temporarily moonstruck Cleo's hand before she touched water): Cleo? You alright?
Cleo: (coming back to her senses) I'm fine, why?
Lewis: We just had quite the close call.
Cleo: Really? (Lewis closes the curtains completely) This is really scary, isn't it? (Cleo stands up) I have to go to the bathroom.
Lewis: Someone should go with you.
Cleo: Lewis, it's the bathroom. I think I'll be fine.

Emma: (Cleo starts singing beautifully after returning from the bathroom) She can sing.

Rikki: Look. Wet hands- but no tail.

Cleo: I can sing! I can really sing. I mean, it sounded good to me, but sometimes it's hard to tell.

Rikki: Cleo listen: you touched water, and the full moon is doing something to you.
Cleo: (nods a little and smiles) I can sing!
Emma: Cleo, wake up! You're a bad singer.
Cleo: (To Emma and Rikki) You guys are just jealous. But that's ok, i still like you! You're my friends! And if you want, you can pick out my next song.

Cleo: Don't you like my singing dad?
Don Sertori: It's nice Cleo, but right now I'm more concerned about you. There are two BOYS down there, who seem a bit...well, smitten.
Cleo: They like me?!
Don Sertori: That's what it looks like. Now, I want you to go down there and tell them to go home right now.

Emma: Reckon we can get any ideas out of Lewis?
Rikki: Forget it; he's a pod person now, like the rest of them.
Cleo: I'm popular! Isn't this great?! (Lewis nods over enthusiastically in entranced agreement)

Rikki: Do you know what happened with Cleo?
Lewis: No, but I think I know what happened to Cleo. She is a siren, a mermaid that used singing to lead sailors to their deaths. Uh, I hope its not that bad.

Cleo: You don't have to do anything! They like me! Adore me; Aren't I adorable? (smiles and leaves)

Cleo (Just after her dad pulled Lewis away mid-kiss): (brushing her pony tail draped over her shoulder, smiling, obviously pleased with herself) You can't protect me forever! (stops smiling as widely).
Emma: No, but we can.
(Cleo shrugs and smiles widely)

Cleo: I have something I want to share with the whole world!

Cleo: You guys can be my entourage. I won't forget you!

Don: Good bye, Lewis!
Cleo: Dad?!




H2O - just add water S1 E12 - The Siren Effect (full episode)

H2O - just add water S1 E12 - The Siren Effect (full episode)