After Belle and Beast got married and are elected king and queen, they established the United States of Auradon, creating a prosperous new nation from the surrounding kingdoms, and banish the villains to the Isle of the Lost, a slum where magic is suspended and which is surrounded by a barrier. Twenty years later, their son, Prince Ben is to reign as the new king. Then, he informs his parents that his first proclamation is to give the Isle of the Lost’s children the chance to live in Auradon, away from the influence of their villainous parents. The first four children he has chosen are Carlos, Cruella de Vil’s son; Jay, Jafar’s son; Evie, the Evil Queen’s daughter; and Mal, Maleficent’s daughter. On the island, Maleficent instructs the quartet to steal the Fairy Godmother’s magic wand to release the barrier so she can take over Auradon.
Traveling to Auradon Prep, the four meet Ben and his self-proclaimed girlfriend Audrey, Princess Aurora’s daughter. They also meet the Fairy Godmother who is the school’s headmistress. Evie uses her mother’s pocket-sized magic mirror to locate the wand in a nearby museum, and Mal uses her mother’s spinning wheel (an artifact in the museum) to put the guard to sleep, but they fail to steal it. After learning that Fairy Godmother will use the wand at Ben’s coronation, the four wait it out by attending classes but start to fit in with the students. Jay is recruited into the school’s “tourney” team (a sport similar to field hockey and lacrosse), while Carlos overcomes his fear of dogs by befriending the school’s mutt, Dude. Evie, though intelligent, acts vain to impress Chad, Cinderella’s son, but ends up doing his homework for him. Dopey’s son, Doug encourages her not to pander to others and be herself.
Mal becomes popular, using Maleficent’s spell book to improve the looks of Jane, Fairy Godmother’s daughter; and Lonnie, Mulan’s daughter. Learning that Ben’s girlfriend will be seated close to the wand during the coronation, Mal bakes a cookie laced with a love potion and gives it to Ben, who falls madly in love with her, much to his friends’ shock. On a date with Ben, Mal becomes conflicted with her inner goodness and desire to please her mother, unsure of how to react to Ben’s feelings towards her. During the school’s family day, the villains’ children are ostracized after an encounter with Audrey’s grandmother Queen Leah. Though Ben, Lonnie and Doug remain friendly towards them, they are forced to distance themselves from the quartet.
On the day of the coronation, Mal gives Ben a brownie containing the love spell’s antidote, believing it is unnecessary to keep Ben under the spell, but reveals he was already freed of the spell since their date when he went swimming in the Enchanted Lake and believing that Mal only did it because she really liked him. During Ben’s crowning, a disillusioned Jane grabs the wand from her mother, accidentally destroying the Isle’s barrier. Mal takes the wand from Jane, but torn over what to do, is encouraged by Ben to make her own choice rather than follow Maleficent’s path. Mal recognizes that she and her friends found happiness in Auradon, and they choose to be good.
Maleficent crashes the ceremony, freezing everyone except herself and the four children. When they defy her, Maleficent transforms into a dragon. Mal and her friends use a counterspell, turning Maleficent into a tiny lizard based on the amount of love in her heart. Mal returns Fairy Godmother her wand as she unfreezes everyone and tells her not to be hard on Jane. While the villains watch the celebration from afar, Auradon Prep’s students party through the night, with Mal and her friends finding happiness.