The Winx will take classes at Cloud Tower this week, so they can understand the dark arts and face Darkar. Musa hates witches, as she hasn't gotten over the time the Trix and other witches attacked her. When they get to Cloud Tower and begin taking the classes, Griffin tells them dark energy comes from mayhem or misery. By focusing on something they hate, they can conjure dark energy. Musa creates a powerful dark sphere by thinking about witches. Stella doesn't even try, creating an orb of light instead, while Bloom, who had a bad week, creates an extremely dark sphere.
The Trix search for Cloud Tower's codex and split up.
The witches refuse to work with the Winx on a project, leading to an argument between them before Mirta breaks it up and they come to an agreement. Mirta and Lucy reflect on previous days at school, and Mirta reminds Lucy of a time when Darcy turned Lucy into a dragonfly and a fairy spell changed her back. Although Lucy has some resentment for Mirta learning fairy magic at Alfea, this doesn't last much longer. Later, the Trix attack Lucy and force her to work for them.
Professor Zarathustra tells the Winx to turn their demons into actual demons by thinking of the times their friends hurt their feelings or betrayed them, until Mirta reveals this isn't part of the course, and Zarathustra is trying to get them to fight each other. Griffin talks to Professor Ediltrude about this, and Ediltrude says Zarathustra did this because she doesn't like the Winx being here. It is suggested that their presence is to provide security for the codex, when Griffin discovers that Lucy is spying on them, but knows someone else is behind it.
Lucy runs into the Winx and Mirta, and tells them that the Trix are here, made her spy for them, and are after the codex. The Winx try to come up with a plan for finding them, and split up. When Tecna tracks the codex, the Trix appear before her. When she refuses to tell them where it is, Icy freezes her, then they use her map to find the codex room, which has a barrier around it. So the Trix decide to take over Cloud Tower.
Bloom and Flora find Tecna frozen. Bloom goes to stop the Trix while telling Flora to find Stella and have her melt the ice.
When the Trix enter the witch lounge, Musa and Layla fight them alongside the witches. The Trix gain control of the tower and bring statues to life to fight them as a distraction.
Bloom reaches the heart of Cloud Tower, where the map says the codex is.
After destroying the statues, Musa and Layla argue with the witches before Flora, Stella, and the unfrozen Tecna break it up.
The Trix reach the heart and Bloom fights them, but they beat her up. As the other Winx and the witches arrive, the Trix open the heart of Cloud Tower. There's a shadow inside.