The Winx Club are back. This time, the Winx will have their journey underwater and will have to face a new dreadful villain, Tritannus. He was a normal triton who was mutated into a powerful monster caused by pollution. His main goal is to rule all the world by entering the Infinite Ocean. The Believix power is not strong enough underwater, as the Believix wings are useless when they become wet hence the Winx start a challenging quest to get the ancient Sirenix power and become Ocean Fairies. Wonderful sea creatures such as the Selkies will help the fairies in this "Underwater Mission", Will the Winx manage to save the Magic Dimension? Will they prevent Tritannus from becoming the evil Emperor of the Infinite Ocean?