It is such a beautiful evening in Gardenia when… the Winx receive a call for a new mission.
Tritannus, an evil aquatic monster, hungers to conquer all the worlds in the Magical Dimension! To bring his terrible plan to life, he has joined forces with the three old enemies of the Winx: the TRIX!
During the first fight against Tritannus, who threatens the inhabitants of the Earth, Sky loses his precious pendant that he wanted to give to Bloom as symbol of his love for her…
With his trident, Tritannus draws evil energy from the pollution of Earth's oceans! The Winx will stop him only with the Sirenix power. Under the guidence of Faragonda, the Winx begin to search for the Sirenix Book so they can learn the secrets of this new power...
The Sirenix Book is guarded in the Magical Archives in Alfea ... But in the great hall full of magical books, there are the Trix waiting for the Winx! They cannot allow the fairies to find the book ...
Specialists are ready to fight side by side with the Winx. But during the conflict with the Trix, Sky gets hurt and loses his memory. His most precious memories fade away, including the love that bounds him to Bloom!
Tecna invents a device that will help her friends Winx find the Sirenix Book but, due to a wicked spell, she turns into a robot! This will not prevent the Winx to save her friend and finally find the Sirenix Book!
Thanks to the courage the Winx have shown in their missions, the six fairies get the Harmonix power, to face submarine challenges and keep searching for the Sirenix power.
Bloom and her friends the Winx receive as gift six Sirenix Boxes: special guardians live within them, and they will help the Winx in the search for the three gems needed to win the Sirenix power.
Luckily, during their adventures, the Winx meet such new special friends: the Selkies, guardians of the underwater portals! After being magically bounded to the Winx, the Selkies will always be by their side and help them to accomplish their mission.
The Selkies find the pendant that Sky had lost in the sea and finally return it to him: Sky regains all of his most precious memories, including those that bind him to Bloom, discovering that their love is a sign of destiny!
Tritannus is more determined than ever: to conquer all the worlds in the Magical Dimension, he weakens the three magical pillars that support the Infinite Ocean. Thanks to their Sirenix power and the precious help of the Selkies, the Winx fight with great courage and defeat their evil enemies.
Daphne, Bloom’s sister, imprisoned by Tritannus, is finally set free by Bloom and her brave friends. The determination of the fairies breaks the curse of Sirenix and Daphne returns to be a girl just like all the others!
To celebrate the defeat of Tritannus, the people of Andros organize a big party! And the Winx give to all of their closest friends a really super concert!