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After the events of Descendants 3, Mal discovers that she has a secret twin sister named Darcy, who was separated from her at birth. Darcy has been raised by the Winx Club, who have been keeping her hidden from the world due to her dangerous powers.
One day, Darcy accidentally reveals herself to Mal, and they quickly become close friends. But their relationship is complicated when Darcy falls in love with Carlos, who is still grieving over the loss of his girlfriend, Jane.
Meanwhile, the Winx Club are being hunted down by a group of powerful villains called the Seliv, lead by Eliva, who want to capture Darcy and use her powers for their own evil purposes. The Winx Club turn to the Descendants for help, and together they embark on a dangerous mission to stop the villains and protect Darcy.
Along the way, there are tons of plot twists and unexpected romantic pairings. Evie and Musa fall in love, but their relationship is threatened by the return of Musa's ex-boyfriend, Riven. Tecna falls for Ben.
In the end, the heroes are able to defeat the villains and save the day. Darcy is finally able to embrace her powers and find her place in the world, and everyone ends up in unexpected romantic couples.