Mandy worked with Emmet to kill their classmates. She then betrays and kills Emmet. Garth is the only survivor and believes Mandy to have saved his life by killing Emmet.
Mandy Lane (Amber Heard) is a very attractive but virginal high-school student. She always seems aloof, is very quiet, and takes great enjoyment in her athleticism. Mandy also lives with her aunt and cousin (the fate of her parents is unknown). Though the cool crowd (especially the boys) seek to date and befriend her, her only close friend is a geeky outcast named Emmet (Michael Welch).
One night, she is invited by uber-jock Dylan to his pool party and she invites Emmet to tag along. Dylan hits on Mandy and is immediately shut down and he takes his frustrations out on Emmet. Embarassed by Dylan, Emmet runs up to the roof overlooking the pool. Dylan goes up to apologize and Emmet taunts him for striking out with Mandy, saying she would only be impressed by a grand gesture. Wanting to prove himself, Dylan (with Emmet's encouragement) jumps off the roof towards the pool to impress Mandy, but he misjudged the distance and breaks his head open on the side of the pool. Dylan dies while Mandy stares accusingly at Emmet.
Nine months later, Mandy appears to have gotten more popular and broken off her friendship with Emmet who is widely blamed for Dylan's death. She agrees to go on a trip to the secluded ranch estate belonging to Red's (Aaron Himmelstein) parents. Red, the school stoner, can't believe his luck that a girl as gorgeous as Mandy would go to his house. They are joined by athletes Jake (Luke Grimes) and Bird (Edwin Hodge) as well as cheerleaders Chloe (Whitney Able) and Marlin (Melissa Price). It seems that all but Mandy have hooked up with each other at some point, although both Chloe and Marlin currently lust after Jake. Chloe and Marlin also constantly insult each other's bodies. Mandy never participates in their insults and seems put-off by their behavior. All three men appear to have their eyes on Mandy and objectify the women.
When they get to Red's estate, Bird goes on a walk with Mandy where he convinces her to hold his hand and let him kiss her on the cheek. Bird begins pressing his luck to Mandy's discomfort. The two are interrupted by Red's ranch-hand Garth (Anson Mount). Garth is an adult and ex-marine who tragically lost his wife and now raises the cattle on Red's ranch. All the girls, including Mandy, appear to find Garth highly attractive. As the night presses on, the teens drink and flirt. Chloe and Marlin insult the size of Jake's manhood, causing him to storm out. Marlin follows him and "cheers him up" in the barn, not realizing a hooded figure is watching them. After they are done, Jake insults Marlin and returns to the house where cuts off the power. While the others work to turn the power back on, he aggressively attempts to hook-up with Mandy, who turns him down equally as aggressively. Jake eventually gives up and restores the power. Mandy then begins flirting with Chloe.
Marlin, meanwhile, is attacked and killed by the hooded figure in the barn. As the night goes on, Jake gets hammered and runs off with alcohol, a gun, and the car to look for Marlin. He finds her dying body at the edge of a lake. Before he can react, he is attacked and shot to death by the hooded figure. The teens hear the gunshots, but assume it is simply Jake acting drunk and foolish. Garth hears the shots and threatens to call Red's parents, but is ultimately captivated by Mandy and convinced not to do so. The kids wait on the porch for Jake and Marlin to return. The hooded killer drives the car past the house, shooting fireworks at them. Thinking it is Jake, Bird chases after the car. He opens the driver's door to discover that the killer is Emmet. The two end up fighting, but Emmet blinds and then kills Bird. Meanwhile, the other three and Garth fall asleep from exhaustion.
The next morning, Garth realizes that an intruder is in the house. They open the door to leave when Emmet shoots Garth through the shoulder. Mandy stays to tend to the now-unconscious Garth's wounds while Red and Chloe sneak out the back to find the car. Mandy gets a first-aid kit and also finds the discarded knife Emmet used to kill Bird. Red and Chloe come across the maimed corpses of their friends before they find the car. The pair kiss (in relief, or having finally realized their attraction for each other) when Emmet shoots Red. Chloe runs back to the house while Emmet kills a now-paralyzed Red.
Chloe sees Mandy outside and runs into her arms... where Mandy stabs and kills her with Emmet's knife. Emmet drives up to the house and hugs Mandy. Emmet then gives two cyanide pills to Mandy, telling her that she needs to shoot him before the pills kill her (revealing they had a suicide pact). Mandy whispers something into Emmet's ears and then kisses him deeply She asks a confused Emmet if that is what he has always wanted. Mandy then explains she has changed her mind about committing suicide and says he never should have done anything for her. Emmet threatens to kill her and Mandy laughs in his face as she notices Garth has regained consciousness and is aiming a gun at Emmet. In his weakened state, Garth misses and Emmet beats him up before chasing after a fleeing Mandy. The pair eventual fall into a mass grave containing the carcasses of diseased cattle. The two fight, but Emmet eventually gives up and Mandy kills him.
Mandy returns to Garth and helps him into the car. As they drive off, Garth says that Mandy is a hero. She gives him a smile and the movie cuts back to a scene earlier in the film where Mandy watches as the classmates she and Emmet have killed happily getting along.
No motive or explanation is ever given for Mandy's homicidal nature. When we learn that she has a role in the killing of her friends, it throws in doubt how "accidental" Dylan's death actually was.
Thanks Evan B!