Stella gets into a fight with Flora, who is working on a potion for repelling witches to protect them from the Trix, and her plants are invading Stella's room, shrinking her clothes and using her makeup. A Nocturnal Ivy kidnaps Tecna and traps her in its vines, also eating her notebook. After Flora releases her, Tecna, Musa, and Stella get angry at Flora for her plants ruining their living conditions. Flora feels bad about the trouble and decides to move to Black Mud Swamp until she finishes her potion. Bloom is the only one who tries to stop her, and gets angry at the others, and gets them to go to the swamp with Flora.
Flora is searching for a one of a kind flower, the Cheerful Gladioulus, for her potion. It is with the water nymphs, the Undines, a.k.a. the Junior League, and plans to clone it so it is not endangered anymore. Stella has a feeling that something bad is out there, and something is - an evil tree, the Windigo Willow.
The Winx find the Undines, whose queen tells them that the island where they get their food is blocked by a water monster attacking them. Any Undines that try to go to the island will be eaten, and one disappeared trying to go to the island. If they can't get to the island, the Undines will starve. The Winx transform to go after the monster.
The monster appears and eats Musa. The other four Winx cast a barrier-like spell with oxygen inside and go underwater, Bloom rescuing Musa from inside the monster's mouth with an extraction spell. On return to the surface, everyone except Tecna falls asleep due to a sleeping gas that is in the air, Tecna using a helmet to keep from breathing it in, and discovers the source to be the Windigo Willow. Tecna casts a barrier around herself and Bloom and removes the sleeping gas from the dome so that Bloom wakes up. They expand the barrier so the others wake up, and power converge to make the barrier cover the entire island.
The missing Undine wakes up and reveals that they're not on an island, but a giant turtle, which is the monster, and she, the turtle, is under the spell of the Windigo Willow, which is a parasite. The Windigo Willow attacks them, and is immune to Bloom's fire magic, but with her help Flora is able to cast a spell to burn the Windigo Willow from the inside, and then destroys him with another magic spell.
The Undine queen says the Windigo Willow was sucking power from swamps for centuries, but now he's gone and the turtle is free from his control. However, there is only one Zillith left so the Undines seem like they have to move. Flora uses her cloning potion on the Zillith leaves to create many more, so the Undines can stay. Stella and Tecna apologize to Flora, who in turn apologizes to them for her plants getting out of control. The Undines reward Flora with the Cheerful Gladioulus.