Cassie (Emily Osment) is a thirteen-year-old goth girl who just moved with her family to a new neighborhood and school. She loves to trick the popular kids at school and her younger brother Max (Alex Winzenread).
On the way to a library, Cassie finds a mysterious Halloween store in an alley and goes inside. The store owner (Tobin Bell) insists on selling her an old book titled The Evil Thing, which contains a warning not to read it aloud or think about its monster called "The Evil Thing", a two-headed creature that sucks blood while the other head eats meat. That night, Cassie ignores the warning in the book and reads it to Max as revenge for him unplugging her computer in the middle of a report she was writing. Priscilla Wright (Brittany Curran), a school bully who stole The Evil Thing from Cassie's bag in school, films her jumping in fear, after using a CD player to play monster sounds outside in order to make the monster appear to have come to life.
After Cassie's parents leave for a Halloween party and later that night, The Evil Thing is brought to life by Max's thoughts, and captures him, Priscilla and a Papa John's Pizza delivery man. It is up to Cassie and Sean (Cody Linley), a popular boy who Cassie likes, to save them. At first, they try to defeat The Evil Thing on their own, but fail. Cassie then asks the owner of the shop, where she bought the book, for help. Cassie discovers that the store owner travels around the world each Halloween, searching for a person who loves to scare people the most and tricks them into reading the book aloud, releasing The Evil Thing, possibly in order to teach them a lesson. He leaves them with the riddle "two heads are better than one; that's the way to get the bloody job done" before his shop disappears. After he disappears, Sean solves the riddle. Together, they formulate that if they get blood from a roast that Cassie's mother made and throw it onto The Evil Thing, it will be forced to suck up the blood. In doing so, it will devour itself. While they plan their attack, The Evil Thing's babies hatch and tries to eat the victims. Luckily, Sean and Cassie make it in time. By using Sean's CD of monster sound effects, they are able to lure the monster to the CD player; however, when the monster nudges it, the music turns into hip hop, thus ruining their plan. While fumbling, Sean and Cassie accidentally drop the blood near Max, who must conquer his fears. When Cassie encourages him, Max tosses the blood onto The Evil Thing, causing its heads to begin attacking each other. The monster eats itself, eventually exploding in a shower of yellow blood, killing itself and its offspring.
With The Evil Thing destroyed, Max, Sean and Cassie save Priscilla. She angrily yells at Cassie and calls her a witch, and Sean rejects Priscilla in favor of Cassie. After Priscilla leaves, they save the pizza delivery man, who offers them free pizza in return for releasing him, then follows Priscilla to hit on her, to her disgust. Cassie and Sean burn the book and set aside all their thoughts about The Evil Thing. Sean leaves shortly afterward. Cassie and Max's parents then come home and find the book in the fireplace. The father mockingly reads it out loud, reviving The Evil Thing. The film concludes with Cassie realizing that The Evil Thing has been brought back to life and that it is not over yet.
What do Sean and Cassie accidentally drop near Max?
Blood