That's not how you do it!
The Winx learn that to defeat Tritannus, they must gain a new power; Sirenix. Bloom's sister Daphne warns Bloom of the dangers of Sirenix. But when looking for the book of Sirenix, another danger threatens Bloom's future.
PlotA new school year begins at Alfea. Headmistress Faragonda warmly welcomes all the returning and new students, especially introducing Roxy . Everyone watches the Winx appear and return to Alfea. The students are very happy to meet the Winx. They reconnect with them. A few of the students also take a photo with Stella. The Specialists hear about the Winx's arrival at Alfea and come to meet them. Later, Bloom informs Miss Faragonda that they have come to consult with her on how to deal with Tritannus as their Believix powers lose effectiveness in the water. Faragonda brings up the matter of the Sirenix power, which could give the girls an edge against the new threat. However, the book containing the secrets is hidden somewhere in Alfea and that even she herself does not know of its exact location. The only one who does is Daphne as she was the last fairy to gain Sirenix. In their dorm, the Tecna is running a search on all known databases on Sirenix but has no luck with Flora saying that Sirenix must be ancient and wonders what the powers are. Aisha gets a call from her mom. She says that her uncle king Neptune is very angry that Tritannus has escaped.
In the waters of Andros, the king is very angry demanding to know how his son got away. Phylla and Lemmy terrified, respond by saying that Tritannus has changed and that he has trned into a monster and that he used his trident on the guards to turn them into monsters just like him. King Neptune is determined to find his son and break him.Back at Lake Roccaluce, Bloom sets out to talk with Daphne and asks about the Sirenix book's location, but the ghostly fairy is reluctant to tell her since the Sirenix is too dangerous to use as she knows firsthand. Daphne relents and tells Bloom it is in the secret archive of the library at Alfea.
In an unknown location in one of the oceans, Tritannus complains about his power being too week and contacts the Trix. Icy informs him of the Winx's arrival at Alfea. She tells him that she has a plan to destroy them. Darcy says that they are too week as the Winx are stronger than they are. So Tritannus makes them stronger by giving them pollution. At Alfea, the Winx with the help of the specialists decide to find the book of Sirenix. As they start look for the book in the archive, the Trix break in and attack the Winx. A fight ensues. The girls have a bit of trouble since the Trix had gotten a serious power-up from Tritannus to overcome their Believix. Icy gets Bloom cornered as she is about to deal out by killing Bloom, Sky intervenes, and takes the hit for her, and falls from a great height. Once Faragonda and Griselda arrive, the Trix retreat. Bloom frantically checks Sky's condition, and is elated to see him awake, but is shocked to know that he doesn't remember her. He has lost his memory when he fell.
- Roxy is introduced to the other students in Alfea college.
- Krystal, the Princess of Linphea is introduced as an old friend of Helia.
- Krystal starts coming between Flora and Helia
- Sirenix is revealed to be an ancient power that was born out of the magic of the sea.
- It is revealed that Daphne was the last known fairy to have acquired Sirenix.
- Neptune hunts for and disowns Tritannus.
- The Trix get more powerful with the help of Tritannus.
- Daphne explains to Bloom that she once acquired Sirenix to help protect Domino against the Ancestral Witches, but they had placed an evil spell on Sirenix, which when Daphne tried using it against the Ancestresses, caused the power to turn against herself and turned her into "a spirit without a body".
- Sky gets amnesia, and forgets who Bloom and the rest of the Winx girls are. This could mean that the prophecy about the Pendant of Eraklyon that Sky mentioned could be true.
- Winx Club
- Ancestral Witches
- Elixir Waves - Used by Krystal to heal Helia after he was attacked by Darcy.
- Fire Arrow - Used by Bloom in a convergence with Morphix Wave against Icy but failed.
- Morphix Wave - Used twice by Aisha, first in a convergence with Fire Arrow against Icy but failed, then against Icy.
- Mega Watt - Used by Tecna against Darcy but failed.
- Solar Storm - Used by Stella against Darcy.
- Super Prism - Used by Tecna to defend Brandon and herself.
- Dark Hypnotic - Used by Darcy against Stella.
- Dragon Heart - Used by Bloom against Icy's attack.
- Morphix Net - Used by Aisha against the dragon.
- Techno Shock - Used by Tecna against Stormy.
- Mirta, Clarice, Ortensia, Alice, Amaryl, Francis, Khadija, Marzia, Emma, Trista and Alice make appearances in this episode; Alice being the only one of them to actually say a line.
- This is, since after being absent during much of the previous season, the Winx, at long last (following 26 Winx Club TV series episodes), make their (long-awaited) official Alfea reappearance and return.
- When Faragonda introduced Roxy, the paw on her top was black instead of blue.
- When Bloom goes to meet Daphne, and when she is talking to her, her bangle is in her right hand instead of her left.
- In one scene, the hearts on Bloom's top are missing when she is speaking to Daphne.
- In one scene, Bloom's bangle is missing.
- When Tritannus gives the Trix more power, the "D" on Darcy is an "O".
- When the Winx are talking about the Sirenix Book, and when Tecna scans the magic archive, her necklace is missing.
- When Sky wakes up after he is shot, he says "What... What happened?", but later says "Who are you?".
“Oh you Winx. You're always the same. But my sisters and I got some new stuff!!”