Estella is a creative youngster with a flair for fashion in 1964 England, but she is bullied because of her odd hair and develops a devious streak. Catherine, her mother, intends to relocate the family to London, stopping at a party at Hellman Hall to enlist the help of the host. Estella steals her mother’s necklace and is chased inside by the host’s fierce Dalmatians, who knock Catherine down a cliffside balcony to her death. Estella blames herself and flees to London, where she befriends street urchins and brothers Jasper and Horace.
Estella practices thieving and grifts with Jasper and Horace ten years later, in 1974, perfecting her fashion abilities by constructing their disguises with their canines, Buddy and Wink. Jasper and Horace get Estella a job at the Liberty department store for her birthday, but she is assigned a janitor and denied the opportunity to exploit her talents. She impresses the Baroness von Hellman—a renowned but dictatorial haute couture designer—by drunkenly redecorating a window display, and she is offered a coveted job at her fashion firm. Estella gains the Baroness’ trust, but she observes that she is wearing Catherine’s jewelry, which the Baroness claims is a family treasure stolen by an employee. During the Baroness’s Black and White Ball, Estella asks Jasper and Horace to assist her in retrieving the jewelry.
To hide her true identity, Estella develops the alter-ego “Cruella” and dresses in one of the Baroness’s old designs from the flamboyant Artie’s vintage clothing business. Cruella steals the show during the ball when Jasper and Horace break into the Baroness’ vault, but she already has the necklace on. Estella swipes the necklace when Jasper throws rats into the celebration. With a dog whistle, the Baroness summons her Dalmatians, and Estella knows the Baroness is to blame for Catherine’s death. One of the Dalmatians swallows the necklace in the ensuing commotion. Estella tells Jasper and Horace to kidnap the Dalmatians and recovers the necklace as a form of vengeance. Cruella dresses extravagantly and upstages the Baroness at several parties, acquiring popularity through Estella’s childhood friend, society columnist Anita Darling. The Baroness fires her lawyer, Roger Dearly, in a fit of rage, while Cruella’s increasingly arrogant behavior irritates Jasper.
Estella creates an intricately beaded dress as the signature piece for the Baroness’s spring collection and plot a burglary in the fashion house, prompting the Baroness to lock up all of the gowns. The Baroness unlocks the vault on the night of the spring exhibition to discover that the garment has been ruined—the beads were actually moth eggs, and hundreds of moths have hatched from them. The Baroness learns Estella and Cruella are the same person after seeing what she has done. Cruella holds her own fashion display outside in Regent’s Park, wearing a counterfeit Dalmatian-fur coat, after wrecking the Baroness’ show. When Estella returns home, she is met by the Baroness and her men, who have kidnapped Jasper and Horace. The Baroness sets fire to the building, leaving Estella to die, and sends Jasper and Horace to prison for her murder. John, the Baroness’s servant, saves Estella by revealing that the necklace unlocks a box carrying Estella’s birth records: the Baroness is her biological mother. To focus on her profession and protect her late husband’s money, she had instructed John to murder the infant Estella. Instead, John handed the infant to one of the Baroness’ maids, Catherine, who nurtured Estella in secret.
Cruella frees Jasper and Horace from prison and discloses the truth, enlisting their help, as well as that of Artie and John, in her last ploy. The quintet sneaks into the Baroness’ charity event, where everyone is dressed as Cruella de Vil. On the balcony, Estella confronts her mother, who feigns an embrace before shoving her off the cliff, unintentionally witnessed by her guests. Estella manages to escape with a hidden parachute and, now that she is legally dead, she takes on the Cruella de Vil persona for good. The Baroness is apprehended, threatening Cruella De Vil with vengeance. Estella willed her inheritance to Cruella before her “death,” including the manor, which she renames Hell Hall and moves in with her accomplices. Cruella gives each of Anita and Roger a Dalmatian puppy in a mid-credits sequence.