Deep in the undersea labyrinth of Andros, the Winx search for the Gem of Self-Confidence; meanwhile on land, the Trix cause another pollution crisis.
Bloom, Stella, and Aisha swim through the ocean of Andros in search of the Cove of Shimmering Shells. They encounter Lemmy, who then bonds with Aisha and regains her powers which were previously stolen by Tritannus. She tells them that Tritannus had indeed passed through the gate. Aisha asks Lemmy to lead them to the Cove of Shimmering Shells.
Back at Alfea, Flora, Musa, and Tecna are going through the virtual ocean simulation to help the other three get to the Cove. Tecna contacts the others to warn them that the place is like a labyrinth and to keep their guard up, which does indeed tie in with the riddle they were given. Bloom, Stella, Aisha, and Lemmy enter the cove, which was filled with pollution. Lemmy falls victim to it, followed by the other Winx girls. Once the oil overtook them, they were separated and each subjected to a personal nightmare. For Bloom, she was met by an image of Diaspro who taunts Bloom that Sky no longer loves her. For Stella, she meets a dark version of herself who tells her that she is nothing but a spoiled talent-less fairy who would amount to nothing. For Aisha, she was shocked to see an image of Nabu, who asks her why she allowed him to die when she could have done something to prevent it. Aisha breaks down from the words, but then recovered, realizing that the real Nabu would not say something like that to her. She stood her ground and repelled the illusion. She figured.
Bloom and Stella were also experiencing the same thing. Aisha reached out to them to fight the illusions. They heard her words, and Bloom tells the Diaspro image that Sky does indeed love her and is confident she will help him remember that, making the illusion vanish. Stella defiantly tells her dark half that she has grown as a person and that she is not worthless, making her vanish. Once the girls were free of the illusions, they rescue Lemmy. The shells covered in oil were indeed the Shimmering Shells and the Winx use their powers to make them shine. The shells were sentient, congratulating them for proving their self-confidence. They give them another riddle to finding the Gem of Self-Confidence, much to Stella's disappointment. According to the Shells, it is located in the dark depths that shine.
Regrouping at Alfea, the Winx were no closer to solving the latest riddle location. Stella then tries to use her Sirenix Box to ask her Sirenix Guardian Spirit to solve it for her. The Guardian replies of course she will not solve it for her, but she gives her a subtle hint when she tells Stella to think about it more, and addressing her as "Princess of Solaria". Stella is disappointed at the answer, but Bloom, Tecna, and Aisha pick up on the hint as the answer having something to do with Solaria. Stella then inadvertently remembered a place on Solaria called the Royal Deeps, where light shined in its dark depths.
Meanwhile, Tritannus goes to invade the oceans of Solaria and heads for the gate, but is stopped by another Gate Keeper, Illiris. Tritannus is not phased and simply drains her power as well before passing through the gate. He then mutates a passing jellyfish. Once he does, he reverts to his normal state. He immediately calls Icy to get him more pollution on Earth. The Trix then pay a visit to Gardenia...
The next day at Alfea, Bloom is with Sky, trying to come to terms about him talking with Diaspro to gain some of his memories back. However, he tells her that his talk with Diaspro was nothing more than a waste of time, that all she could talk about was herself, and that she was really boring to him. Bloom laughs at this, her fears about them are put to rest. She then gets a call from Vanessa, who tells her that something is polluting the coasts of Gardenia. Bloom, Musa, Tecna, and Sky go to Gardenia to see what was going on, while Stella, Flora, Aisha, and Brandon go to Solaria to find the Gem of Self-Confidence.
At a local waste facility, the workers had been hypnotized by the Trix to dump barrels of toxic waste and oil into the water, causing Tritannus to regain his demon form. Bloom's group meets up with Vanessa and Mike, who tell them about the situation. The oceans suddenly starts to become dark with oil. They quickly head for the source.
At Solaria, Stella's group use their Harmonix to explore the ocean and run into Lemmy, who was also accompanied by Illiris. She and Stella bond and regains her powers which were stolen by Tritannus. The Keepers lead the girls through the Azure Valley and into the Royal Depths. They also find the place filled with pollution and quickly advance to escape it. In the central cove, they find a silver shield that reflects light from the sun into the depths, which answers how there is light down there to begin with. Suddenly, the Winx get ambushed by the mutated Jellyfish from earlier. During the fight, Flora gets Lemmy and Illiris out of harm's way of the jellyfish, but then it petrifies her to stone, much to the horror of Stella and Aisha.
At the waste facility, Bloom's group manages to sneak in and notice something wrong with the workers. The Trix suddenly appear and attack. Bloom, Musa, and Tecna activate their Harmonix and fight the Trix while Sky deals with the helpers. To the Trix's shock, the Winx were easily manhandling them. They whine about how the fairies are not supposed to be stronger than them.
Unfortunately, due to his amnesia, Sky has also forgotten how to fight. When Bloom dove in to help, she gets blind-sided by Icy. Musa manages to repel the sea demons and Tecna stopped the oil outflow. Icy, Darcy, and Stormy then unleash a combined assault to kill Bloom who was still recovering. Seeing Bloom in danger awakened a memory in Sky, in which he jumps in and destroyed the combined attack spell with his sword, shocking the Trix. Bloom then attacks the Trix, causing them to fly back. The three witches run like the cowards they are once they realize the fight was not in their favor. The workers were freed of the hypnosis, and Bloom assures them that they will set things right.
At the Royal Deeps, Stella and Aisha were still tangling with the jellyfish monster. Thinking fast, Stella takes the silver shield and taunts the monster to shoot at her. It does, and she uses the shield to reflect the beam back at it, causing it to petrify into stone by its own attack. Once it was petrified, Stella reverts Flora out of being petrified from stone, by undoing the jellyfish's spell, reversing her body from stone to normal and moving again. Aisha asked Stella how she knew that it would work, and Stella admits that she did not, and merely took a chance, and even Flora thanked her for turning her back to normal again. She also discovered the Gem of Self-Confidence in the shield's center. Suddenly, their Sirenix Boxes, which had a red gem and orange gem on two of its points, were now joined by a blue gem. Three gems down and two to go. Bloom's group meets up with Mike and Vanessa, who were very pleased to see the oceans clear again. The Trix complain to Tritannus that their powers currently were no match for the Winx's new Harmonix powers. He reassures that there is a way to overcome it, and all they need is Bloom's sister, Daphne.
- Aisha and Stella quickly emotionally bond with the selkies of their own respective home worlds, Andros and Solaria.
- The Winx Club succeed in gaining the mystical Gem of Self-Confidence.
- Sky regains a part of his memory, the memory of how to fight.
- Ice Spiral - Used by Icy against Musa.
- Reverberating Notes - Used by Musa against Icy.
- Storm of Numbers - Used by Tecna against Stormy.
- Impenetrable Darkness - Used by Darcy against Tecna.
- X-Ray Vision - Used by Tecna to see through Darcy's Impenetrable Darkness.
- Fire Blade - Used twice by Bloom, first against the Trix, then against the mutant.
- Diapason - Used by Musa against the mutants.
- Globitronic Wall - Used by Tecna to block the oil from flowing to the ocean.
- Dragon's Embrace - Used by Bloom to defend Sky and herself.
- On the Nickelodeon website this episode is called "The Magical Union" and is an alternative title.
- It was reuploaded onto Nickelodeon site and called the original title of the episode.
- Since in Italian, the word "medusa" can refer both to a jellyfish and to the Gorgon Medusa who had the power to turn everyone who looked at her directly into stone, the scene where Stella reflects the petrifying beam of the mutant jellyfish and turns it to stone may be a reference to the legend of Perseus and Medusa in Greek mythology where Perseus looked at Medusa's reflection on his polished shield to behead Medusa.
- This episode debuts Flora, Musa, and Tecna's Harmonix transformations.
- A member of the gang seen in Gardenia in Season 2 is seen on the cover of a magazine Flora is reading.
- In one scene, when Bloom was fighting with the Trix, her Harmonix wings do not have the hearts effects.
- When the Sirenix Boxes and the gem appears, Flora's wings have hearts effects instead of flowers effects.
- When Bloom was battling the Trix, for a few seconds part of her hair were missing.
- When Flora was turned into a statue, her prototype version of Harmonix hairstyle was used instead of the final version.
“He forced the gate, and used his trident to steal my powers!”
“Another reason to despise that guy.”
“Sky doesn't love you, Bloom. Heh, if he loved you, he wouldn't forget. Hah! He WOULDN'T forget! Ha-ha-ha-ha!!”
“So, you wanna be a fashion designer? Never. Going to happen. It takes talent and that's something you just don't have. Haha! Never going to happen. NEVER going to happen.”
“Then why did you let me die? You let me die. You could've saved me but you let me die.”
“Venture where the dark depth of the ocean shine. A precious gem, self-confidence, lies in the deeps.”
“How many hits did you get? A billion?”
“No. Only fifty one million nine hundred and sixty seven thousand four hundred thirty eight.”
“Royalty, of course! The Royal Deeps, where the water is deep and dark but is filled with light!”
“Her master's voice.”
“The funny thing is, when we go to talking... I realized she's really boring! The only thing she talks about is herself!”