H2O: Just Add Water - Season 3 - Eps 8: Kidnapped
During a full moon, the girls stay at Cleo's and Bella is attacked by the water creature. At the same time, Will decides to return to the moon pool after making the connection between his previous encounter with the moon pool waterfall and the timing of the full moon.
About H2O: Just Add Water

Title: H2O: Just Add Water
First Air Date: 2006-07-07
Last Air Date: 2010-04-16
Status: Ended
Rating: 7.812/10 (from 653 votes)
Language: EN
Seasons: 3
Total Episodes: 78
Network: Network Ten
Genres: Drama, Family, Sci-Fi & Fantasy
Production Companies: Pacific Film and Television Commission, Jonathan M. Shiff Productions, Network Ten, ZDF
Synopsis
H2O: Just Add Water revolves around three teenage girls facing everyday teen problems with an added twist: they cope with the burden of growing a giant fin and transforming into mermaids whenever they come in contact with water.
Cast

Phoebe Tonkin
Cleo Sertori

Cariba Heine
Rikki Chadwick

Indiana Evans
Isabel "Bella" Hartley

Luke Mitchell
Will Benjamin

Burgess Abernethy
Zane Bennett