After the Sirenix book bestows the power of Harmonix, the Winx set out on their first quest: a hunt for three gems, starting with self-confidence.
Following the events of the previous episode, Tecna and Aisha try to open the Sirenix Book but discovered that it will not open because they do not have the power to. Musa and Flora comes in to tell them that Faragonda wants to speak with them.
Meanwhile outside in the courtyard, Bloom continues to attempt to jog up Sky's memories. As it looks like she is finally reaching him, Princess Krystal arrives to tell Sky she has brought someone who might possibly be able to help restore his memories. To Bloom's shock, it is Diaspro. Diaspro immediately pulls Sky away to "reacquaint him" about his past. An oblivious Krystal looks pleased with herself, thinking that bringing Diaspro would help, which Stella makes a sarcastic remark about. Stella and Bloom are not happy about the turn of events but they do not show it. Stella tells Bloom the others are waiting for her in Faragonda's office.
In Faragonda's office, the headmistress tells the Winx that they must complete the quest of Sirenix in one lunar cycle or they will lose their powers forever. This is not a decision to be taken lightly, and that in order to truly attain it, all of them must agree to the decision. She then tells them to go to Graynor to represent Alfea and compete in a little competition with other fairies of the Magic Dimension.
The girls arrive at Graynor where other fairy teams have already been situated and meet the Ancestral Spirit of Nature. She will grant the winning fairies Nature's Boon which would increase their powers dramatically. The contest is to search for the Creature of the Rainbow Mantle. She warns that the creature is elusive and will only show herself to those who are worthy. Stella, Musa, and Tecna search the mountains while Bloom, Flora, and Aisha search the riverbed areas. Unknown to them, Tritannus was spying on them and sends the Trix to finish them off.
In the mountains, Tecna is filled with self-doubt due to the robot incident in the previous episode, which she ties to her upbringing on Zenith where it is revealed that Zenith inhabitants do not show their emotions, making her wonder how the others can even trust her anymore. Stella and Musa put her fears to rest, saying they trust her wholeheartedly. Musa calls a few birds down with her singing. When asked about it, she replies it was something her mother taught her. Tecna mentions that Musa hardly talked about her mother, and she replies that maybe she should start now.
At the riverbed area, Bloom, Flora, and Aisha have no more luck finding the Rainbow Mantle Creature. Flora then conjures up some enchanted strawberries to attract magical creatures. Instead, she winds up attracting a monster the Trix summoned. The girls quickly transform to fight it off. While they are busy with it, the Trix go off to find the Creature of the Rainbow Mantle and kill her so that no one will get the Nature's Boon.
Back with Stella, Musa, and Tecna, they find a stag with rainbow-colored horns. One of the fairy teams had been following them and tried to get it. Tecna does not care since she believes that the stag is not the creature they were looking for. Suddenly, a horse shows itself, disappointing the girls. The stag runs off, but the Trix arrive to attack. The fairy team shoots at them, which has no effect, and are evidently scared out of their minds. Just as the Trix are about to kill them, Stella quickly blocks the attack, and a fight ensues.
Unfortunately, due to Tritannus' power-up, the Trix are too strong and knock the three Winx out. In the fight, the horse from before is knocked off the cliff and is hanging for dear life onto a branch. As per her sick and twisted nature, Icy decides to drop the horse to its death for fun, but is stopped by Bloom, Flora, and Aisha, who arrive in time. The branch broke, causing the helpless horse to fall, and Bloom quickly saves it with her powers. She sets it on the cliff and tells it to stay clear from the fight. The three are hard-pressed trying to fight the Trix and protect the unconscious Stella, Musa, and Tecna, and it is not going so well. The horse then suddenly rushes past the two opposing sides and reveals itself to be the real Creature of the Rainbow Mantle, healing the Winx. The Trix go in to kill her off, but the Rainbow Creature easily repels them, forcing the three witches to retreat. The Winx return back to the Ancestral Spirit of Nature with the Rainbow Mantle Creature, winning the competition and the right to Nature's Boon.
Back at Alfea, the girls use their powers to open the Sirenix Book. A voice from the book tells them that this is the start of their long and perilous journey, and that if they cannot attain Sirenix within a lunar cycle, they will lose their powers forever. Their task is to find the Gems of Self-Confidence, Empathy, and Courage, which are hidden somewhere in the vast oceans of the Magic Dimension. They are each granted Sirenix Boxes, containing guardian spirits of Sirenix. The voice then leaves them a riddle to finding the first gem.
Tecna, Musa, and Flora stay behind at Alfea to research on the Cove of Shimmering Shells, which is somewhere on Andros, but no one knows where it is. Bloom, Stella, and Aisha go to Andros to find the cove, going on directions found by Tecna. The three girls then activate their new power of Harmonix to begin their search and in the pollution, they spotted Lemmy.
- The Winx learn that to achieve Sirenix, they must complete their quest in one lunar cycle or they could lose their powers forever if they fail.
- The Winx open the Sirenix Book and they each receive a Sirenix Box, in which stay their Sirenix Guardians.
- The Winx achieve Harmonix from their Guardians of Sirenix.
- Tecna, Flora and Musa ask Palladium for help about finding the Shimmering Shells.
- Bloom, Stella and Aisha transform into Harmonix Fairies for the first time.
- Creature of the Rainbow Mantle
- Bloom, Stella and Aisha Harmonix
- Ancestral Spirit of Nature
- Nature's Key
- Sirenix Boxes
- Cove of the Shimmering Shells
- Odyssey Explorer
- Winx Club
- Royal Crewman
- Alfea Students
- Alfea Staff
- Dragon Heart - Used in a convergence with Andros Hurricane against the monster.
- Andros Hurricane - Used three, twice against the monster, then against the Trix.
- Spring Ring - Used to defend herself but failed.
- Morphix Staff - Used against the monster.
- Fire Arrow - Used twice, first against the monster, then against the Trix.
- Autumn Wind - Used twice, first on a convergence with Andros Hurricane and Dragon Heart against the monster, then on a convergence against the Trix with Morphix Wave
- Voice of the Forest - Used to find the Creature of the Rainbow Mantle.
- Solar Storm - Used against the Trix, but failed.
- Sonic Screen - Used to defend herself, Tecna.
- Sirius' Shield - Used to defend herself, Musa and Tecna.
- Morphix Wave - Used on a convergence with Autumn Wind and Fire Arrow against the Trix.
- Dragon Wing - Used to save the Creature of the Rainbow Mantle from falling off the cliff.
- Morphix Net - Used against Icy.
- Shadow Hand - Used against Flora.
- Icicle Barrage - Used against Bloom, Flora and Aisha.
- Tecna mentions her home planet for the first time since Episode 4 in Season 1.
- This is the first time Tecna came up with portable means of communication.
- It is revealed that people on Zenith do not show their emotions.
- Musa reveals her opera singing talent for the first time.
- This is the first time we hear the Harmonix song.
- On the Nickelodeon website, this episode is titled: "The Hidden Rainbow".
- It was re-uploaded on the Nickelodeon website a few days later and is now called The Power of Harmonix.
- Even though the Winx earned their Harmonix powers, the Harmonix transformations are only shown for Bloom, Stella and Aisha.
- This is the last appearance of the Believix transformation. This is also the first time Believix was really weak for the Winx.
- This also marks the last hearing of the Believix song.
- This is the third time a new transformation was earned during the sixth episode of the season. The first being Enchantix, the second being Believix.
- Diaspro appears for the first time since Episode 9 in Season 3.
- In the original/Italian version, the Winx have more than a month to complete their missions.
- The title of the episode - "The Power of Harmonix" - sounds similar to the tag name of Season 4 in the Nick Dub - "The Power of Believix".
- After the Ancestral Spirit of Nature's speech, Bloom's boot overlaps Flora's boots.
- Many times, Bloom's eyes are green instead of blue.
- Bloom's bangle is missing when they are discussing whether to open the Sirenix Book or not.
- Aisha's bangles are missing when the Ancestral Spirit of Nature was leaving.
- Musa's necklace is missing when she and Stella hug Tecna.
- Bloom's bangle is on her right instead of left hand.
- When Musa, Stella and Tecna faint, Musa is on the right of Stella and Tecna on the left but when the Creature of the Rainbow Mantle cures them, Tecna and Musa swap places.
- There are times where Flora's inner wings are a solid pink instead of the usual fading colors of yellow to orange to pink.
- At Alfea when the Winx earn the Sirenix Boxes Bloom is standing in the middle of Tecna and Flora, but when the light comes from the book and gives them the Sirenix Boxes, Bloom is standing in the middle of Flora and Musa.
- When the Creature of the Rainbow Mantle rushes past the Trix and Winx, Icy's glove is missing
“I've known you for years, we grew up together!”
“So, you could tell me about Eraklyon?”
“Of course, it's my home too!”
“So, the ex-fiancee returns...”
“Tecna, I'm dying here! And you don't even care!”
“But I do care! I care a lot! I'm sorry if I seem cold, at home on Zenith people don't share their emotions. But after what happened in the Magic Archive... I turned into a robot! How can you ever trust me?!”
“We trust you, Tecna.”
“With our lives, if we have to.”
“The quest for Sirenix begins. But know this: there is a curse upon the Sirenix. You must complete the quest within one lunar cycle or you will all lose your powers. Forever! To gain Sirenix, you must find gems of self-confidence, empathy, and courage; which are hidden in the oceans of the Magic Dimension. ...For each of you, a Sirenix Box. Your guardian of Sirenix resides within. You may call on her for help during your quest. ...Your guardians have granted you a new fairy power: Harmonix. It will allow you to swim faster and use powers freely in the ocean. The quest begins!”
“These are hazardous waters. Think I should go down before you.”
“We can handle ourselves!”