While the world celebrates the spectacle of the rare comet, Cleo, Rikki, and Bella know it's destined to strike earth and set out to stop it. Bella locates an ancient tale from when the comet last passed about how a tower of light shot from the earth, touched the comet and saved them from cataclysm. On hearing that a young girl named Eva was responsible for the light, the girls realize the Water Tentacle's potential. Meanwhile, in an effort to recover more unique crystals, Sophie and Ryan damage the Moon Pool's ability to create the magical energy the mermaids need, just so they can be rich. Although they end up not keeping the gems anyway, because Will presented Sophie an ultimatum: gems or him. The girls combine their powers to create their own tower of light and risk everything in an attempt to direct it toward the comet. They succeed and prevent a collision, saving everybody and fulfilling their destiny as mermaids. Later, Rikki, Cleo, and Bella graduate from school to enter a new phase of their lives, forever bound by the sea and their shared secret. Lewis returns and Cleo asks him why he came all that way to see her. Lewis jokes that he heard that "there's a new science genius in town" and he wanted to "check out the competition". Rikki and Zane again become friends. During the celebration at the café, Bella sings the shows opening and closing theme song "No Ordinary Girl".
- Angus McLaren returns as Lewis McCartney.
- This is the last episode of Season 3, and the last episode to air in 2010.
- This is the last episode of H2O.
- It is revealed that Bella's real name is Isabella at her high school graduation.
- One graduating student is Lucy Fry, an actor of H2O's spin-off, Mako: Island of Secrets.
- Phoebe Tonkin and Cariba Heine are the only actors who appear in every episode of each season.
- This is also the final episode with the Rikki's Café in it.
- This episode aired on June 13, 2012 in United States, over two years after the show ended in it's home country other parts of the world. It aired part of H2O's Finale Week on Teenick.
- Featured Song: "No Ordinary Girl"
- Final episode directed by Colin Budds.
- Rikki: It's a piece of paper, really.
- Cleo: You shouldn't be celebrating this, you should.
- Sam: We should be what?
- Sophie: That was before we found out the cave was loaded with this stuff. I say we get all we can.
- Zane: Some things are more important than money.
- Will: I love you sis, but if you take these, you lose me. And I don't want to lose you.
- Bella: Guys - what if the tower of light wasn't light - what if it was water?
- Cleo: They've ruined the Moon Pool. All the magic's gone.
- Rikki: I never thought of you as a nerd before. Wow!
- Kim: Nothing exciting ever happens around here.
- Sam: That's not right. This shouldn't be happening.
- Don: No, she's finished school, she's earned it.
- Sophie: So, you don't need me to watch over you then.
- Lewis: Oh, I heard there was a new science genius in town, and I wanted to check out the competition.
- Zane: That was a joke, right?
- Bella: Wow, in public and everything. What happened to the shy guy I used to know?
- Kim: If I have to go, so does she.
- Will: 300 years ago, people still thought the earth was flat.
- Ryan: I don't feel right about this, we don't know what kind of rock structure supports this place. Anything could happen.
- Ryan: Oh no. I knew this was a mistake.
- Don: Just don't be late for your graduation day ceremony tomorrow.
- Lewis: I will let you know when I need to breathe.
- Ms. Taylor: Lucy Fry?
- Ms. Taylor: Isabella Hartley.
- Rikki Chadwick - Cariba Heine
- Cleo Sertori - Phoebe Tonkin
- Bella Hartley - Indiana Evans
- Will Benjamin - Luke Mitchell
- Zane Bennett - Burgess Abernethy
- Nate - Jamie Timony
- Kim Sertori - Cleo Massey
- Sophie Benjamin - Taryn Marler
- Samantha Sertori - Penni Gray
- Ryan Tate - Andrew Lees
- Ms. Taylor - Mouche Phillips
- Jason - Ben Timony
- Lead Guitarist - Jarreau La Castra
- Bass Player - Hamish Orr
- Donald Sertori - Alan David Lee
- Lewis McCartney - Angus McLaren
- Lucy Fry - Lucy Fry (uncredited)