Following the events of Descendants: The Royal Wedding, Uma has taken on the role of headmistress and principal of Auradon Prep. Her first major initiative is to open the border between Auradon and Wonderland by inviting Princess Red, the rebellious daughter of the tyrannical Queen of Hearts, to attend the school.
Red and her mother travel to Auradon, where the Queen of Hearts and Cinderella reunite for the first time since their days at Merlin Academy, the original name of Auradon Prep. The reunion quickly turns hostile as the Queen recalls a traumatic prank from their school days. During the opening ceremony, as Uma welcomes everyone, the Queen unexpectedly launches a coup with her magical deck of cards, taking the attendees hostage. When Cinderella refuses to bow to the Queen, a reluctant Red is forced to condemn her to treason by beheading, which delights her mother but angers the crowd. Overwhelmed with horror and guilt, Red tries to use a magic pocket watch to go back in time and prevent the coup by stealing her mother’s deck of cards. However, she is attacked by Cinderella’s perfectionist and kindhearted daughter Chloe, accidentally activating the watch and sending both girls back to when Auradon Prep was still called Merlin Academy. Believing they were sent for a reason, Red and Chloe begrudgingly agree to work together for the sake of their mothers.
Posing as exchange students, Red and Chloe enroll at Merlin Academy, where they meet the teenage versions of Cinderella and the Queen of Hearts, known then as Ella and Bridget. They begin to seek ways to change the future. As they gather information, they learn that Bridget hangs out at the Black Lagoon. Red and Chloe overhear a plan to give Bridget a cupcake that will turn her into a monster during the dance, using a sorcerer’s cookbook. Red and Chloe manage to find the cookbook in Merlin’s study, navigating through his defenses. Although they successfully retrieve the book, the villains are able to take it from them. Since the book should not fall into the wrong hands, the villains are magically frozen, and Merlin discovers the situation, giving Red and Chloe detention. Red hides the cookbook, and the two return to the present.
Back in the present, they arrive at the moment Uma is giving her speech. The Queen of Hearts begins to say the same lines as before, but Red is relieved to see that her mother is wearing a lighter gown and creates heart-shaped bubbles for the celebration, signaling that their actions have changed the course of events.