It's Halloween in the Hollywood Hills, and Kat Dyer's going to have the sexiest costume for a night of shameless debauchery. However, Kat's plan is derailed when the costume shop screws up her order and gives her a costume that is the polar-opposite of sexy. Deciding it's better to be temporarily ugly and seen than beautiful and quickly forgotten, Kat attends the biggest party of the year, only to awaken with the strangest post-Halloween hangover ever: partygoers are transforming into the likenesses of the costumes they wore the night before. Meanwhile, a mysterious force begins to stalk and pick them off on by one. Kat and her friends must discover the cause of the horror, and what they find will haunt them forever.