Penelope Sweetwater Prissily, a beehive-wearing, tantrum-throwing brat, is on a mission to become the 'Meanest Girl in The World.' Also known as Petroleum due to her excessive use of hair gel, Penelope, along with her panicky, moth ball-scented mother, Hattie Prissily, is determined to achieve fame and fortune by crafting an illusion of success.

However, their plans are disrupted when a mischievous, street-performing, broom-selling ghost moves into the neighborhood, captivating audiences with unbelievable magical tricks using a mysterious satchel of marshmallows. The ghost's presence threatens to expose Penelope and Hattie as ordinary and unremarkable, a fate they dread even more than a shortage of hair gel. Determined to uncover the secrets of the ghost's magical powers, Penelope and her friends, the Cinders, embark on a daring mission to steal the ghost's bag of marshmallows and follow him into an abandoned train station.

What ensues is a hilarious and thrilling adventure, as the forces of good and evil engage in a climactic battle over the grandeur of dreams and the enchanting power of marshmallows.