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.
A little later, in the sorority's shower room, Claire and Jessica discussing about the incident. After they leave, a sorority girl named Joanna, who overheard their conversation, gets murdered by the mysterious killer with the same tire iron.
That night, the large graduation party is thrown. Claire's boyfriend, Mickey, is attacked and murdered by the hooded killer, with Ellie witnessing it.
Cassidy, Claire, Jessica and Ellie regroup and all receive a text containing the video of Megan's death and a message telling them to go to the steel mine in 20 minutes or the video will be sent to the police. The girls drive to the mine shaft, and encounter Garrett, who has cut his wrists. Thinking Garrett is the one stalking them, Jessica runs over him with her vehicle. However, they discover afterward that Garrett has been receiving the same text messages they have. Ellie suspects that Megan is the killer, believing that she did not actually die and is seeking revenge. To prove that Megan is dead, the girls lower Cassidy down the shaft to check, but instead of finding Megan's body, she finds finds a message written in blood says "Theta Pi must die".
Back at the now empty sorority house, the girls receive a text from Chugs' cell phone, telling them that she is dead. Afterward, Claire is killed. Searching the house for Jessica's boyfriend, Kyle, the girls bump into Maggie and their house mother, Mrs. Crenshaw (Carrie Fisher). After discovering about the incident, Mrs. Crenshaw tells the girls to lock themselves in a bedroom and to call the police while she searches for the killer, who is in the house. Maggie is shocked and leaves the room to find Megan. After a game of cat and mouse in the kitchen, Mrs. Crenshaw is killed. The girls hear the shots from Mrs Crenshaw's shotgun and believe that she killed Megan. With no cell phones around, Cassidy and Jessica decide to find Mickey's body and use his cell, telling Ellie to run if she gets the chance. Downstairs, the killer confronts Maggie and throws a Molotov cocktail at her, setting the house on fire.
Cassidy and Jessica find Mickey's body but run into Kyle, who injures Jessica after getting into a fight with her. Cassidy and Jessica flee to an under-renovation bathroom where they find Megan's decomposing corpse hanging in the shower. Kyle attacks them there and knocks out Jessica. Before he can harm Cassidy, Kyle is killed with an axe by Andy, Cassidy's boyfriend, who reveals himself to be the killer. Andy at first protected them by keeping the knowledge to himself. Since Andy wanted to have a perfect future with Cassidy, but she was always at risk of being found out. So, Andy decided to kill everyone who knew of Megan's murder. Andy then kills Jessica and tells Cassidy that they need to kill Ellie too.
Cassidy is opposed to this, but Andy says that the geekish Ellie is the weakest and reveals that Ellie was the one who confided in him all about Megan's death, so she will be likely to tell someone else. Devising a plan, Cassidy plays along and tells Andy that Ellie is in the basement, and while Andy goes to look for her, Cassidy fetches Ellie from upstairs and they try to run out the front door, but are attacked by Andy, who is hurt that Cassidy would betray him. Urging Ellie to flee, Cassidy hits Andy with a lamp and tries to escape, but stops as she hears the cries for help of Maggie, who is trapped in the flames. Cassidy tries to help her out but Andy comes and tries to kill Maggie, but Cassidy stops him. The floor then crumbles underneath Cassidy, and she is left hanging over the burning basement. As Andy is about to finish her off, Ellie appears and shoots him with Mrs. Crenshaw's shotgun. He falls backward onto the burning floor, which collapses under him and he falls to his death into the flames. With Andy dead, Ellie tells Maggie to wrap a curtain around herself and run through the flames, the two then pull Cassidy back up from the hole in the floor. The three survivors, Cassidy, Ellie, and Maggie escape from the burning sorority house, just as the fire department and other emergency personnel arrive.
15 months later. Theta Pi is being renovated from the fire and Maggie is now a Theta sister. As the girls sing the Theta Pi song, a man comes into view, holding a garden trowel. The camera pans up to his wrist, revealing fire burn scars.