Air Date:
21 March 2008
Directed by:
Colin Budds
Written by:
Colin Roache
Preceded By:
Followed By:

Unfathomable is the 26th and final episode of Season 2 of H2O: Just Add Water. It was originally meant to be the series finale.


The final full moon of the season approaches. It will be Charlotte's first one, and to protect their secret, the girls try to warn her. Charlotte angrily retorts that she's "more mermaid than the three of them put together" and thrusts them into the air before dropping them and giving them bruises just as Ash appears. Charlotte is convinced that Lewis made a mistake in choosing Cleo over her and is determined to remove the girls' powers at all costs. That night, while the girls are sealed inside Emma's house, Ash comes over. Tired of the mysteries surrounding Emma, Ash demands an explanation from her, but Emma hesitates. Charlotte purposefully looks at the full moon to make her full of strength and revenge, and begins causing havoc and mayhem in Emma's house by using her powers to control the water and make it spray everywhere. The girls are powerless to retaliate since Ash is in the house and they are not yet comfortable with revealing their secret to him. As Charlotte clearly planned, Lewis asks her to go to Mako Island; Cleo finds out and follows them there. Emma and Rikki then follow to help her. When the three girls are all together at Mako Island, Charlotte and Lewis reveal that this full moon can permanently remove a mermaid's powers and that it is how her own grandmother, Gracie, became an ordinary girl again. The girls battle each other; while Charlotte is stronger than each of them individually, the three girls work as a team to strike down her creations (water snakes), and make her plan backfire on her. As Charlotte falls into the moon pool as the full moon passes over, she is now no longer a mermaid. The next day, Lewis convinces an ashamed Charlotte to give the locket back and return it to its rightful owner, Cleo. Rikki asks Lewis if Charlotte will reveal the girls' secret and Lewis explains that she won't; she gave him her word of honor. Ash, to his surprise, finally discovers Emma is a mermaid and reacts in an overjoyed way to the news. This season ends with Ash, Zane, and Lewis happily watching the girls run into the ocean and swim off as mermaids.

Alternate Episode Title

It is listed under the title Unfathomable on Netfilx and re-runs on Disney Channel Australia.


  • This is the final episode with Claire Holt as Emma, Brittany Byrnes as Charlotte, and Craig Horner as Ash in it.
  • This is the last episode of Season 2, and the last episode to air in 2008. 
  • This episode was supposed to be the final episode of H2O, but it was continued after it received an increase in its audience and fan-base.
  • Trent Sullivan as Elliot, Jared Robinson as Neil, and Caroline Kennison as Lisa are credited in the episode, but they don't appear in the episode.
  • This is also the final episode with the JuiceNet Café in it.



  • Song Featured: "You're Everything"





H2O - just add water S2 E26 - Unfathomable (full episode)

H2O - just add water S2 E26 - Unfathomable (full episode)