The Winx Club embark on a mission to achieve Sirenix, an ancient transformation, that can be used in the water, on land or in the air, unlike believix which can only be used on land of in the air. It is strong enough to defeat the evil merman Tritannus and the Trix. The Trix's main leader, Icy falls deeply in love with Tritannus, while the two other Trix members-Darcy and Stormy- are frustrated and angry with their romantic relationship. There are two new evolutionary fairy transformations in this season: Harmonix and Sirenix. The ancient Book of Sirenix gave them six Sirenix Boxes that holds guardians inside them to help them gain Sirenix and much, much stronger Harmonix abilities and spells. Before they achieve Sirenix, however, they must find the Gem of Self-Confidence, the Gem of Empathy, and the Gem of Courage; otherwise they would lose all of their magical powers forever.
Unfortunately, there is a dark and evil curse upon Sirenix that left Bloom's older sister, Daphne, a yellow-colored disembodied spirit without a physical body of solid flesh and blood. But there is a Sirenix book that helps them fully gain access to Sirenix, but there is a problem when they open the book. For instance, Tecna opens it and it transforms her into a terrifying robot version of her self, making a mess in the Magic Archive the library in Alfea. When she had used its tremendous power to defend herself. Daphne was the last fairy to achieve Sirenix's ocean/sea-based transformation and enormous power until the Winx Club gain full access to Sirenix and cross the gate into the infinite ocean itself. Finally, the Winx gain Sirenix after activating the source of the power in Lake Roccaluce where Daphne lives due to her connection to Sirenix. But Daphne gets locked up in a prison cell under the mighty Emperor's throne after Tritannus has taken away her Sirenix powers and given them to the Trix-Icy, Darcy and Stormy-as a witch-themed version. Unfortunately, the Trix gain dark Sirenix with the help of Tritannus ( because he took away Daphne's Sirenix with his trident ) and plan to defeat the Winx Club. Tritannus is also son of king Neptune turned into evil form of his merman form. His tail also has purple pollution track but doesn't stay. He refuels by taking in toxic pollution ( oil, gas, trash, garbage, and acid ) with his trident from the help of Icy and her two sisters.
Roxy will not be a main character in this season, but will only be appeared as a minor character. She will just be a student at Alfea. But she will appear in future seasons and episodes; such the sixth season. Upset that he cannot activate the tremendous, boundless and limitless power of the Emperor's Throne, Tritannus must take the mystical seals of the pillars of light, balance and control. Tritannus stole the seal of the Pillar of Light, and Stella restored the pillar by concentrating and thinking of the ones who makes her strong, those being her parents, with that she had unleashed her special Sirenix spell,Light of Sirenix. Tritannus then stole the seal of the Pillar of Balance. He had almost failed when the Winx tried to ambush both him and Icy, but thanks to Stormy and Darcy controlling the Singing Whales to attack the Winx, they we're able to escape. However, they did not thank the other two Trix, much to Darcy and Stormy's dismay. Musa saved the whales by singing to them using her spell Voice of Sirenix. With two seals gone from two pillars, the next is the pillar of control to finally activate the emperor's throne. To prevent Tritannus from activating the throne, the Winx Club decided to destroy the seal of the pillar of control. After finding out that the seal is destroyed, Tritannus plans to steal Politea's(an old and best friend of Daphne's) Sirenix energy and abilities to activate the throne.
However, he is not the only one who is looking for her: Darcy and Stormy are done with Tritannus and want her tremendously powerful Sirenix energy and abilities for themselves, but Bloom is also looking for her to find a way to break the Sirenix curse on both her and Daphne. Darcy and Stormy succeed and leave Icy. Tritannus plans a big attack on the Winx at the Pillar of light. Luckily the Winx are not alone, they get helped by Tressa, Nereus and the selkies. Tritannus kills Nereus with his trident, but is resurrected thanks to Aisha's wish. While distracted Tritannus kidnaps Aisha and brings her to the throne, but is followed by Bloom. Tritannus promises Icy that she will soon be empress, he than steals Aisha's Sirenix abilities to activate the throne. He becomes completely corrupted and overwhelmed by the almighty and limitless power of the Emperor's throne and attacks Icy, but is protected by her sisters. She breaks up with Tritannus. Bloom finds out that the key to defeating Tritannus is destroying his trident, she does this by using her Fire of Sirenix ability. Defeated, Tritannus is sent to an yet unknown location as punishment for his actions. Bloom uses her Sirenix wish to have Daphne's physical body finally restored. The season ends with a concert from the Winx Club on Andros and with Bloom happily dancing with Daphne.
So Sad And Happy!