Taking place after Magical Adventure, a new danger emerges in the form of Tritannus, Aisha's merman cousin who turned into a monster after absorbing toxic pollution and allied with the Trix. Since their Believix power does not work well under the sea and is not strong enough to fight Tritannus and the Trix, the Winx will have to go on a quest to find the power of Sirenix. However Sirenix is cursed and it has something to do with the past of Daphne, Bloom's elder sister. Aisha also meets Roy, a specialist towards whom she is attracted. This season mainly focuses on environmental care. Roxy is a minor character in this season.
The events of season 5 are followed by those of the third movie, The Mystery of the Abyss.