Tritannus, Aisha’s cousin and a prince from Andros’s underwater kingdom, escapes from prison after trying to kill his brother Nereus. With the help of the Trix, the Triton starts stealing the magic of the Selkies, who guard the underwater gates that connect all the worlds. His goal is to reach the Infinite Ocean and control the Magic Dimension. The Winx must earn a new transformation, Sirenix, and defeat him.