After discovering that her boyfriend Garrett cheated on her, sorority sister Megan (Audrina Patridge), with the help of her sorority friends - Cassidy (Briana Evigan), Jessica (Leah Pipes), Ellie (Rumer Willis), Claire (Jamie Chung) and Chugs, Garrett's sister, pulls a prank on him; she fakes her own death while making out with him. With the help from the sisters, Garrett brings Megan to a steel mill, intent on dumping her body in the lake. After Jessica jokingly mentions they need to release the air out of her lungs so her body will not float to the top, Garrett stabs Megan through the chest with a tire iron, killing her for real. The group decides to dump Megan's body in the mine, along with the tire iron.
Eight months later. The girls have grown apart and put the incident behind them, although Cassidy still feels guilty about the whole thing. During the party held after graduation, the girls all receive a picture sent to their phones: a robed arm holding the bloody lug wrench Garrett used to kill Megan. Suspicion falls on Garrett, but Chugs insists he's changed after the incident and is not mentaly competent. Jessica takes the lead and insists it is merely a sick joke by him and continues with the party. Megan's young sister, Maggie, arrives, wanting to honor her sister's memory by attending the party.
Later, Chugs leaves to go to her therapist appointment. Not knowing that her therapist was killed, but instead thinks he is playing a sexual game, she lies down and drinks champagne when she is suddenly murdered by a mysterious killer wearing a hooded black robe and uses the spiked tire iron.