Plot
At a Texas high school, Mandy Lane blossoms over the summer, attracting her male classmates. One of them, Dylan, invites Mandy to a pool party at his house. She accepts with the provision that her best friend Emmet can come along. At the party, Dylan bullies and humiliates Emmet, until Mandy intervenes. As revenge, Emmet convinces a drunken Dylan to jump from the roof into the pool, but Dylan fails to reach the pool, smashing his head on the concrete, which kills him.
Nine months later, Mandy has since befriended many of Dylan's popular friends, while Emmet has been subjected to even more intense bullying. Their stoner classmate Red plans a weekend party at his father's remote ranch, and Mandy reluctantly accepts an invitation from Chloe, a popular but insecure cheerleader. Mandy accompanies Red and Chloe, along with several other classmates—including reserved football player Bird, and romantic couple Jake and Marlin—to Red's ranch. Upon arriving, they are introduced to Garth, the ranch hand.
That night, Jake gets offended over a joke and storms off to a nearby barn, where Marlin performs oral sex on him. They have another argument and, after Jake walks back to the house, an unseen assailant knocks Marlin out and breaks her jaw with the barrel of a shotgun. Back at the house, Jake unsuccessfully attempts to woo Mandy. Anxious about Marlin, Jake then takes Red's shotgun and pickup truck to go search for her. He eventually finds her sitting by a remote lake. Upon closer look, he sees her mangled face, and is confronted by Emmet, seeking vengeance for the humiliation he has suffered. Emmet shoots Jake in the head and breaks Marlin's neck, killing both.
Emmet drives back to the ranch in Red's truck and sees the rest of the group on the porch. He shoots fireworks at them. Bird gives chase, believing the driver to be Jake, playing a prank. Emmet confronts Bird and attacks him, eventually slashing his eyes with a knife and stabbing him to death. The rest of the group, drunk and high, fall asleep at the house along with Garth.
The next morning, as the group leaves out the front door, Emmet shoots and wounds Garth. While Mandy tends to Garth, Red and Chloe try to run to Chloe's car. Emmet shoots Red and chases after Chloe. In Garth's shack, Mandy retrieves the keys to his truck and finds the bloodied knife that Emmet used to kill Bird. She goes outside to find Chloe being chased in her direction. Mandy embraces Chloe, but then stabs her in the stomach, revealing that she is in league with Emmet.
As Chloe bleeds to death, Mandy and Emmet discuss the suicide pact they had planned. Mandy reveals she had no intention of going through with it, convinced that Emmet agreed to the murders only on the basis of winning her affection. Refusing to let her back down, Emmet prepares to shoot her, but Garth intervenes by wounding Emmet with his shotgun, prompting Emmet to stab him multiple times. Emmet chases Mandy into the fields, where they fall into a ditch filled with cattle carcasses. Mandy grabs a log and defends herself against Emmet's machete, eventually killing him. She returns to an injured Garth and they drive away from the ranch. Assuming she was also merely a victim of the carnage, Garth thanks Mandy for saving his life.
A flashback shows the group back at a railroad track, where they took a break from their drive. While the rest goof off, Mandy balances on the tracks and watches her future victims.
