The film opens with a car full of friends led by Brandon Meeks (Edward DeRuiter) with his pal Tony and their girlfriends cruising and vandalizing the streets of Blanca Carne when the group decides to order food at a fast food restaurant parody of McDonald's called "Hella-Burger". However, while ordering they are repeatedly insulted through the intercom system. Furious, Tony breaks into the restaurant gun-loaded "to show clown boy what's up". While inside, Tony continues his search for the employee, a concerned Brandon also enters the restaurant, (leaving both Brittany and Tiffany in the car), only finds Tony dead with his face peeled off after friend the deep fryer; in a state shock Brandon is then attacked by a demonic version of Hella-Burger's mascot "Horny the Clown". Brandon tries to bargain with Horny but to no avail as he is killed. Horny then makes his way outside and gruesomely kills both Brittany and Tiffany in the car. Meanwhile, after a house party, local teenagers Mackenzie Carpenter (Leighton Meester) and her boyfriend named Fisher Kent (Nicholas D'Agosto) play with a Ouija board, asking "What's going to happen to our pathetic little lives?". The board, obviously moving of its own volition, repeatedly spells out the message "N1KLPL8". The meaning of the message is unclear until the next day, when Mackenzie and Fisher find out that the murdered teenagers were riding in a car with the license plate number "N1KLPL8". The news bulletin also reveals that no dead bodies have yet been found by the police.
At school, Mackenzie was upset that someone seems to have stolen her camera at the party, and that she needed pictures for the school year book. Later, while playing with a Magic 8 Ball, she asks it if she should "Get baked in the bathroom or go to class". To her dismay, the Magic 8 Ball responds "Hi Mackenzie". She then asks "Who are you?" to which the Magic 8 ball responds only "You'll see". Finally she asks "Why are you doing this?" the Magic 8 ball responds "A Broken Heart" which is later implied to be referring to a new tattoo worn by Mackenzie's best friend Val. After school, Mackenzie stays so she can finish her work on the school year book, but suddenly the janitor named Lenny (Sean Whalen) arrives and returns Mackenzie's camera, not explaining where he found it. As Mackenzie develops the photos in the school darkroom, she finds photos depicting the gruesome murders of the four teenagers (consists of Brandon Meeks, Tony, Brittany, and Tiffany) that was previously slaughtered by Horny in revenge for insulting him on the intercom system. She is then chased through the halls by Horny the Clown, and witnesses the death of her friend named Val after Horny chained her head in a modified microwave, and finds Lenny hanging dead in the boys bathroom before finally running into a police officer. However, the police cannot find bodies of Val and Lenny, and due to Lenny's criminal record, they discount Mackenzie's testimony and believe that Lenny himself murdered the four teenagers before hanging himself out of guilt.
The next day, Fisher goes to Mackenzie's house right before the town carnival is about to begin. Fisher is seen playing with an Etch-a-Sketch, but at Mackenzie's request he puts it down, and after some discussion, the two have sex for the first time on Mackenzie's bed. Unknown to them, the Etch-a-Sketch spontaneously writes out the phrase "I ♥ BUSH". Later, at the Carnival, Tina McCandless, is shown wearing a t-shirt with the same "I ♥ Bush" phrase, just before she and her boyfriend named Chad Baldwin are killed, murdered, and slaughtered by Horny in the haunted house. Again, no dead bodies are found, and the only witness is Fisher himself, who is in shock after seeing Horny the Clown but otherwise unharmed, and is taken to the local hospital. Mackenzie discovers that her mother used to be friends with the parents of Brandon Meeks, Val Espinoza, and Tina McCandless. At the hospital, Mackenzie stays in for the night until Fisher gets better. While getting up to get a drink of water, a vending machine drops out a toy with Horny's face on it with a message, saying "See you at 4:20". Mackenzie goes to show the message to Detective "Crackers" Crockers (Larry Joe Campbell), who has been keeping watch on Fisher. She finds the detective lying on the floor, asleep, with a burger in his hand, and Fisher missing from his bed. Fisher put his prescribed tranquilizer into the burger while Crockers wasn't looking, and Crockers (who has been established as a habitual overeaten) subsequently ate the burger and passed out.
Mackenzie gets in her car and Fisher scares her. Mackenzie and Fisher discovers that the 4:20 message is standing for the 4:20 Boyz led by Chuck Taylor (Shedrack Anderson III) along with his friends named Spanky and Carrie, (as it turns out Chuck's father was the boyfriend of Marcia Carpenter) .Later that night Chuck, Spanky, and Carrie are stoned and stop by Hella-Burger, while Carrie orders the food, both Spanky and Chuck play at the fun space zone. Horny then emerge from the ball pit area and Horny killed Spanky by killing him vertically with his trademark meat cleaver and Horny stabbing him repeatedly with his cleaver. Carrie survives, however, being in the right place at the wrong time and again no dead bodies are found. When Mackenzie arrives home and her mother, Marcia explains why the children of her friends are being killed in a gruesome manners.
There was a boy named Archie Benjamin (Van De La Plante), who was the son of Jack Benjamin. They used to push him around like a scapegoat and joke with him every time they went there, but despite that he was best friends with Mackenzie's mom, Marcia Carpenter. On the night of his 18th birthday, he invited Marcia to come to his party, but they instead smoked weed. Charlie didn't like Archie at all because of friendship with Marcia, so he convinced the rest of the group to play a prank on him, which involved Charlie wearing the Horny mask and using a meat cleaver to scare him. Charlie accidentally knocks Archie out cold also knocking over the table carrying the birthday cake, causing a fire, Marcia and the group quickly flee leaving Archie to burn to death.
Mackenzie realizes that Archie somehow came back from the dead, Believing that Jack Benjamin (John Gilbert) the father of the late Archie might be able to stop him, Mackenzie, Fisher, and her friends: Starfire (Rachael Bella) and Van (Penn Badgley) go over to Mr. Benjamin's house. Mackenzie and Fisher go talk to him, while Van goes to check the backyard, and Starfire goes to check the garage to see if there's any body parts in the ice cream freezer. Looking around for Mr. Benjamin, Mackenzie and Fisher find Archie's room, and they find a bunch of toys in his room, implying that Archie is sending messages in the toys he used to have.
Meanwhile, Van enters the garage to look for Starfire. He finds an ice cream meat freezer, then he finds Starfire's severed corpse in there. Horny then appear behind him and Horny slashes Van in half. Horny then re-enter the house and chases Mackenzie and Fisher. Horny corners Fisher by making Fisher's eyes turn bloody red. Meanwhile, Detective Chase (Lola Glaudini) and Detective Crockers come to the house because Marcia told them where Mackenzie was going. Chase finds Jack Benjamin behind his own bed, trying to hide himself from Archie and Heather. Meanwhile, Mackenzie wakes up in the local Hella Burger, tied to a chair, with duct tape on her mouth. There is a birthday cake in front of her, and she's surrounded by the corpses of Brandon Meeks, Val Espinoza, Tina McCandless, and Chuck Taylor, but they are set up like a birthday party since it's Mackenzie's 18th birthday. She screams in horror and then Horny enter the restaurant, surprising her. Mackenzie asks where Fisher was and Horny screams "HE WASN'T INVITED!".Horny pours a gallon of gasoline on her and is about to use a birthday candle in order to burn her alive. Suddenly, Marcia Carpenter bursts in shoots Horny in the mouth which she thinks killed them. Marcia unties her daughter's hands before Giggles comes from behind and strangles her. Mackenzie using the distraction to her advantage grabs and drinks a bottle a whiskey on the nearby table but she doesn't consume it, as Horny knocks Marcia out. Horny taunts Mackenzie by holding the birthday candle close to her face with Heather watching. Mackenzie then uses the whiskey in her mouth to blow the fire back into Archie's face, Mackenzie unties herself, knocks Heather out, and Archie is burned alive and apparently dies. Detective Chase tells Detective Crockers to take Archie's mask to have it analyzed. Detective Chase then tells Mackenzie that Fisher is still alive, but he's in critical condition, so Mackenzie and her mother go to the hospital to see him. Meanwhile, Fisher wakes up from his coma, but his eyes are still bloody red, revealing that Archie has possessed his body. As he wakes up, a shadow is seen up against the wall behind him. As Mackenzie and her mother enter the room, Fisher is gone, and the clothes he was wearing are outside on the ground. Meanwhile, Detective Crockers is at Hella-Burger drive thru ordering a large amount of food, the intercom speaker then replies "You wanna die FAT ASS?!" as Horny jumps on top of Crocker's vehicle and Horny murders him through the windshield all while saying "Have a hella nice night!". However, the death of Archie Benjamin was only just the beginning.