After Musa faces off her personal nemesis at Notre Dame, the team reunites to capture three escaped mermaids wreaking havoc in Paris.
Meanwhile, in the Enchanted Swamp, Aisha breaks Tecna and Flora as well as the Crocodile Man and Alligator Man out of the cave's mud using her magic. Once safely outside the caves, Jim and Smee return from the forests but the Crocodile Man remains apprehensive towards them. Jim clarifies that it was the Queen who imprisoned his brother, not them, and Flora flies in between them, claiming that they must put their personal issues past each other now that they are on the same side against the Queen. The Crocodile and Alligator Men agree to join forces with the Winx and Pirates and, with everything now resolved, Tecna believes that they can return to Earth to continue the search for Peter Pan's son. However, after hearing this, Jim accuses the Winx of wasting their time chasing "an impossible dream" as the World of Dreams continues to suffer, but Flora is certain that Tinkerbell will become the good fairy she once was once she sees Matt. Jim reluctantly admits that the Winx may be right in their plan but still thinks that it is too much of a gamble as they do not even know where Matt could be.
Back in Paris, Musa barely manages to dodge another one of Banshee's supersonic screams and runs to the top of the highest roof to try and confront her. Luckily, Bloom and Stella arrive just on time to help but are rendered useless when Banshee blows them off of the Cathedral before Musa could warn them of her attacks. Musa charges in to try and attack Banshee but her magic does not do anything and she leaves herself wide open for retaliation. Letting out another one of her supersonic screams, Banshee knocks Musa into the roof but one of the wooden tiles flies onto one of the Cathedral bells, causing it to ring out and give her pain. Musa quickly takes notice of this, realizes that Banshee's weakness is harmonious melodies and uses her magic to control the Cathedral bells, using them as large instruments to further torment Banshee with their melodies until she explodes.
With Banshee gone, Musa rushes to Bloom and Stella's side to inform them of the good news. As they try to figure out what happened, vines spring out from under the leaves they are resting on and gently lower them down to the ground, revealing that Flora, Tecna and Aisha have returned from their mission in Neverland. However, just as they are about to continue their search for Matt, they are whisked back into Neverland by the World's Spirit, who claims that Neverland needs their help once again.
The Spirit explains that three mermaids who resided in the Mermaid's Lagoon escaped to Paris after Tinkerbell cursed it, turning it into the bleak and murky place that the Winx see in the image the Spirit shows them. She goes on to explain that the mermaids can assume human form and make it a habit to charm people just to have fun, but the Earth cannot house them, so the Winx must drive them back into Neverland where they belong.
Immediately after the briefing, the Winx are warped back to Paris and Stella takes the lead, claiming to have an idea. Though, unbeknownst to them is that Tinkerbell and the Shaman have already sent out one of her many shadows to spy on them. Tinkerbell reflects on how Bloom and Musa were able to defeat their Nemeses but believes that Stella will not be able to get off so easily. The Shaman naturally agrees as Tinkerbell audibly wonders what her weak point is. The Shaman then lists some of Stella's defining traits like her optimism and seemingly boundless energy and Tinkerbell makes a snide remark over how it seems like Stella only has good qualities.
Once the Winxmobile is parked, Stella takes it upon herself to teach the Winx about how mermaids feel and act so that it would be easier to find the ones they are looking for. Tecna proposes that they do so from inside the Winxmobile to avoid being rushed by eager fans, but Stella insists that they need to go out and search the city and claims that they can hide within the crowd to avoid being noticed altogether.
And so, thanks to Stella's affinity for fashion, the Winx each don on tailormade disguises and explore the city of Paris in search of the trio of mermaids. As the six fairies walk around, Stella brings up how mermaids love being admired and suggests that they search for groups of girls who all cannot go without being admired or fawned over. The Winx trail closely behind as Stella weaves through the crowd in search of three girls in smart, classy and attention-grabbing outfits. She quickly comes across three girls who fit her mermaid qualifications but, after seeing how the girls avoided stepping into a nearby puddle, Stella dismisses the idea of them being mermaids.
Continuing on, Flora spots three girls and believes them to be the mermaids they are looking for due to their cheerful and carefree attitudes. Flora's suspicions are almost confirmed when one of them giggles after being splashed by water from a bubbler but, when the other springs from her seat with an annoyed look on her face, Flora quickly drops them from their list of potential mermaids and becomes discouraged. Bloom then asks Stella what a mermaid's real interests are and Stella goes on to list that they are charming, enchanting and love being praised; all of which seem to fit her more than anyone else. Aisha even comments on the similarities which Stella does not find to be funny until a dark-skinned woman with large purple hair passes by her to meet with two of her friends by a fountain in the center of the square.
Regrouping with her friends, the one with brown hair and red highlights talks about how everything around them is new and nice, and specifically mentions how it is nothing like "the lagoon."
Seeing the interesting-looking girls, Stella whispers to the Winx that she has spotted the mermaids. The three girls in question have now hopped into the fountain and are engaged in a water fight, splashing each other with water as they playfully giggle while the surrounding crowd stares in awe and intrigue. With the whole crowd's eyes on them, the girls coax them into joining their game, their eyes turning a suspicious purple color as they do so. Suddenly, everyone in the crowd finds themselves compelled to approach the fountain, making it impossible for the Winx to pass through.
The mysterious trio of girls' fun is put on hold once local officers rush in to get them out of the fountain, however, the girls leap into the waters of the fountain and disappear in the purple mist. This also causes the crowd to disperse and go about their normal routine, all while the officers search for the girls, confused as to where they could have disappeared to.
That is when Aisha approaches the fountain, places her hand on the water and, using her new Onyrix powers, is able to see that the girls used the water to teleport to a party somewhere else in Paris. Stella then begins to rattle her brains trying to figure out where the mermaids have gone to and quickly comes to the conclusion that they are now at a pool party after catching sight of a nearby billboard advertising one.
Later, once evening hits, the Winx arrive at the pool party where the mermaids have gone. Stella insists that their missions would be a lot more fun if they were like this one, but Musa just wants to find the mermaids and leave as quickly as possible, as she finds the place to be too crowded and the music to be terrible. The Winx then split up to search the crowd on their own.
In another part of the party, the dark-skinned mermaid charms a pair of boys she is dancing with into getting her something to eat. They quickly return, each with a plate of sponge cake, and the brunette mermaid takes one of the plates, believing that her friend would never finish both slices on her own. The blonde mermaid catches sight of a man drinking a strawberry smoothie and tells him of how much she absolutely adores them, to which the man replies by suggesting that she go to the counter to get one. Instead, the blonde mermaid approaches the man and repeats herself about how much she adores strawberry smoothies. This time, she charms the man into giving him his smoothie; all of which Stella sees and becomes increasingly suspicious of.
She then takes it upon herself to approach the trio of mermaids and ask if she can join in on their fun by making up a lie of being unable to find her friends. That is when the dark-skinned mermaid comments on Stella's "scratchy" voice, going so far as to claim that it almost pierced her eardrums, and the surrounding group of people begin to laugh. Stella brushes this off with a nervous chuckle and insists that she needs to scream over the loud music if she wishes to be heard. Unfortunately, the mermaids do not let up on Stella as the brunette one claims that she wishes the music were louder so that they would not have to hear Stella's voice. The surrounding crowd bursts out laughing once more, further hurting Stella's confidence, but she does not let that get to her and continues trying to grow closer to the mermaids by insisting that they go dancing. This only makes the mermaids laugh harder as they cannot believe that she would think they would go dancing with her. The blonde mermaid asks Stella rudely if she had even taken a look at her clothes, causing her, her friends and the charmed men to belt out in laughter, even to the point where Stella finds herself speechless as she is thrown into a painful childhood memory of two girls bullying her, using the exact same words as the blonde mermaid.
The girl in red goes on to point out Stella's "awful" appearance, ranging from her shoes to her hair, which even causes the younger Stella to nervously look at one of her pigtails. She tries to shove all of this aside by telling the two that she only wanted to eat lunch with them, but the girl in red dismisses this by asking if Stella is an idiot and makes it clear that she and her friend do not wish to eat lunch with a loser like her. The two of them then walk off, leaving Stella alone to cry, bringing Stella back to reality where she is crying alone as the mermaids pass by her, still laughing.
From her mirror, Tinkerbell catches sight of this and expresses relief over finally uncovering Stella's weakness.
Stella quickly recollects herself after hearing Flora call out to her, asking if she is okay, and Stella turns to meet Bloom and Flora to tell them that she found the mermaids. Unfortunately though, she lost sight of them after they passed by while she was overtaken by her emotions, but Bloom quickly finds them on the other side of the pool dancing as most of the party-goers crowd around them, charmed by their mermaid magic.
The three fairies soon regroup with Aisha, Musa and Tecna near the mermaids and find that they are also unable to get closer. Musa asks what they should do now and, after noticing the accumulating clouds, Aisha uses her magic to make it rain. The sudden downpour takes the crowd out of their trance and they quickly disperse, believing the party to be over. Once the mermaids notice their adoring crowd leaving, the dark-skinned mermaid charms a few men and dives into the pool, transforming into her true form. Her two friends quickly follow suit and the three of them charm the whole crowd into joining them inside the pool.
Musa fears that the rain may have only made the situation worse, but Stella does not thinks so and reveals to the Winx that the mermaids' charming powers are just too strong.
As the mermaids continue on in their fun, they take notice of one of the party-goers having really nice diving skills. Each of the charmed party-goers tries to outdo the others in diving skills until one of them leaps onto the gurdur holding up some party lights and falls into the pool from there. Impressed, the mermaids have the charmed crowd mimic this man, but the combined weight of the party-goers begins to cause the gurdur to shake and lose ground. Tecna wonders if the crowd has gone bonkers until Bloom chimes in to claim that they need to step in before someone gets themselves hurt.
As the party-goers continue on with their crazy dives, the dark-skinned, white-finned mermaid swims over to the DJ and charms him into playing better music. Once he does so, the mermaid returns to her friends and charmed crowd as the light-bearing gurdur begins to crack apart and give way. To prevent the entire crowd from being electrocuted, Flora uses her magic to try and support it with giant vines, however, she cannot hold it up for very long. Bloom shouts over how they need to get everyone out of the pool but Stella does not think they will be able to do so, as the crowd would not listen to them while they are still under the mermaids' charms.
That is when a portal into the World of Dreams opens up from within the depths of the pool, releasing the Crocodile and Alligator Men to terrorize the crowd into fleeing, freeing them from the mermaids' charms. The Winx quickly take notice of the brothers coming to help them while the mermaids are displeased with them and the Winx ruining their party. They then swim over to another part of the pool to escape with the fleeing crowd while the gurdur finally gives way and falls into the pool. With everyone safe, the Winx decide to pursue the mermaids but not without thanking the Crocodile and Alligator Men for their help.
The mermaids continue to flee from the pool party but are quickly met by the Winx suited up in their spy outfits. The dark-skinned mermaid tries to use the water from a nearby fire hydrant to blast them away, but Aisha easily dispels the water, causing the mermaids to realize that the Winx are fairies. The brunette in red suspects that they were sent by the Queen to force them back into their old Lagoon and they try to flee again, only to be startled by an oncoming truck that luckily stops in time. The truck driver hops out of his truck to ask if the mermaids are crazy but the startled mermaids fun off, only to be scared by more and more cars that almost run them over. The Winx pursue the mermaids, who now weave through a crowd and are stopped by a man's opening umbrella. They continue running away, being scared by other things like a barking dog and a man in a sandwich suit, and stop by some crossroads which also happen to be near the Winx.
Tecna uses her magic to switch the stoplights from red to green to prevent the mermaids from crossing the street, giving the rest of the Winx an opportunity to catch up. Their chase soon makes it onto the rooftops of buildings, where the blonde mermaid slips during one of her leaps and almost plummets to her doom, but is luckily caught by one of her friends. As she is being pulled up, Bloom demands to know if the mermaids have realized that they have only gotten themselves into trouble and claims that Earth is not their world, but the dark-skinned mermaid tells Bloom to go tell her Queen that they will never return to the Lagoon as she and her two friends continue to flee. She then picks up the blonde mermaid onto her back after she tells them that she hurt her ankle, but they quickly lose their footing and slip off of the roof!
Now hanging by the breaking gutters, Flora rushes to their aid, only for the gutters to completely snap, causing the mermaids to plummet to the sidewalk below. Fortunately, Flora finds a bundle of vines nearby and uses them to catch the mermaids just before they hit the ground and lowers them to safety. Realizing that the Winx saved them, the mermaids apologize for causing so much trouble as they thought that the Winx were the Queen's allies. The brunette mermaid agrees to the idea of returning to Neverland to seek out Jim and offer their help to their cause. Bloom insists that they will have a better chance at fighting the Queen with more allies and the mermaids thank the Winx for all they have done before leaping into a fountain to return to Neverland as promised.
With the mermaids gone, the Winx can return to their search for Peter Pan's son, Matt, and Musa takes it upon herself to utilize her Sound Harmony spell to track his voice to his current location but not before warning the Winx that she will need absolute silence for it to work. With all of them quiet, Musa uses her Sound Harmony and quickly picks up Matt's voice. Using this, Musa leads the Winx to an apartment and the six fairies break in through one of the ceiling windows.
Once inside, the Winx look around the room to look for Matt and come to realize that Musa had tracked his voice coming from his answering machine to his flat. Realizing that Matt seems to be a solitary person, Musa admits to the fact that it will not be easy tracking his voice in all of Paris. Little do they know is that Matt is out for a walk in the city listening to music as a suspicious man seems to be following him from closely behind.
- The battle between Musa and Banshee continues.
- Aisha frees herself, Tecna, Flora, the Crocodile Man and the Alligator Man from the cursed prison using her power over fluids.
- The Crocodile Man and Alligator Man join forces with the Winx, Jim and Smee against Tinkerbell.
- Musa finds out Banshee's weakness and uses the bells of the Notre Dame Cathedral to defeat her.
- The Spirit of the World of Dreams informs the Winx of a trio of mermaids who have fled to Earth after their lagoon was cursed.
- The Winx begin their search for the mermaids in Paris while Tinkerbell uses one of her shadows to follow the Winx and search for Stella's weak point.
- The Winx find the mermaids but they escape. Luckily, thanks to Aisha's powers, they learn that the mermaids can use bodies of water to teleport from place to place.
- The Winx track the mermaids to a pool party where Tinkerbell finds Stella's weak point after she is bullied by the mermaids.
- With the help of the Crocodile and Alligator Men, the mermaids are driven out of the party and the Winx give chase.
- The Winx rescue the mermaids from certain danger so, as thanks, the mermaids join their cause and return to Neverland.
- Thanks to Musa, the Winx manage to find Matt's flat.
- All throughout the scene where Musa uses the bells of the Notre Dame Cathedral to defeat Banshee, she plays the bells to the tune of the theme song, Winx Club Sparkles of Light.
- As Aisha frees herself and her friends from the mud of the cursed cave, her Onyrix is the same color as Tecna's instead of being its usual slate blue.
- One of the men charmed by the mermaids has a darker skin tone as the brunette mermaid teases Stella on wanting the music to be loud enough to completely mask her "scratchy" voice.
- Louise is seen within the crowd of fleeing party-goers in Paris despite supposedly still being in Gardenia with Annabelle.
- Bloom's eyes are gold instead of blue in one shot when she and the Winx chase after the frightened mermaids.