Run by Mrs. Crenshaw (Carrie Fisher), the college sorority Theta Pi is home to seniors and best friends Jessica (Leah Pipes), Cassidy (Briana Evigan), Ellie (Rumer Willis), Claire (Jamie Chung), Chugs (Margo Harshman) and Megan (Audrina Patridge) who, despite Mrs. Crenshaw's best efforts, constantly throw wild parties. During one of these parties Megan discovers her boyfriend Garrett (Matt O'Leary) is cheating on her and, with her friends' help, decides to pull a prank on him as payback, faking her own death while making out with Garrett. With help from the sisters, Garrett brings Megan out to a steel mill, intent on dropping her body down a mineshaft. After the sisters (jokingly) mention they need to release the air out of her lungs so her body won't rise to the top, Garrett stabs Megan with a tire iron, actually killing Megan. Panicking, the group dumps Megan's body in the mineshaft, vowing never to speak of the events that transpired ever again.
Eight months later, during the party held after graduation, the girls all receive a picture sent to their phones: a hooded figure holding the bloody tire iron Garrett used to kill Megan. As the girls become increasingly worried, Chugs reveals that after the incident, Garrett has been seeing a therapist and has developed a weird sense of humor. Jessica takes the lead and insists it is merely a prank by him and continue with the party. Megan's sister Maggie (Caroline D' Amore) also arrives, wanting to honor her sister's memory by attending the party. Chugs leaves the party to go to her therapist appointment. While she is there, not knowing that her therapist was killed, she lies down and drinks champagne. As she tilts her head back to let the champagne flow down her throat, the killer appears and shoves the bottle down her throat, before eventually shattering the bottle inside her throat with the tire iron, which has been "pimped out" by the killer.
Later, a sorority girl named Joanna is killed in the sorority shower after hearing about Megan's death. In the basement, Ellie finds Cassidy's blood-stained jacket (which the group had wrapped Megan's body in before dropping her into the shaft) while Claire's boyfriend, Mickey (Hennard), is murdered. The girls regroup and all receive a text message containing a clip of Megan's death and a message telling them to go to the mineshaft. Before leaving, Cassidy finds her boyfriend Andy (Julian Morris) and tells him to leave the party and that she'd meet up with him the next day. Driving to the shaft, the girls find Garrett. Thinking the obviously delusional Garrett is the one stalking them Jessica runs over him when he tries to attack Cassidy with a mirror shard, discovering afterward Garrett has been receiving the same messages they have, proving he is not the killer. After the girls return to the now empty sorority house, they receive a text message from Chug's cell phone, telling them that she is dead. While struggling to get into the house, Claire is dragged into the massive amount of bubbles that were coming from the hot tub and shot in the mouth with a flare gun, and Mrs. Crenshaw is impaled, leaving only Jessica, Cassidy, Ellie and Maggie.
While Maggie becomes trapped when the killer sets the house on fire with a Molotov cocktail, Cassidy and Jessica decide to check Mickey’s body for a cell phone to call for help (while Ellie hides upstairs) and run into Kyle (Matt Lanter), Jessica's boyfriend, who injures Jessica after getting into a fight with her. Cassidy and Jessica run to an under-renovation bathroom where they find Megan's corpse hanging in the shower. As Jessica checks the bathroom door, Kyle swings an ax through it, knocking her out. He catches Cassidy and tells her that Jessica told him they murdered Megan. He's then killed by Andy, who turns out to be the real killer. Andy kills Jessica by stabbing her in the mouth before telling Cassidy that he just wanted to help her, killing everyone who knew about her involvement in Megan's death so they could never tell anyone else and reveal she was involved. Playing along with her crazed boyfriend, Cassidy 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, but are attacked by Andy. Telling Ellie to flee, Cassidy hits Andy with a lamp and tries to escape, but stops to help Maggie out of the burning house, allowing Andy to catch up and leave her dangling from a hole in the floor. As Andy prepares to finish Cassidy off, Ellie appears and blasts him with Mrs. Crenshaw's shotgun. With Andy dead, Ellie pulls Cassidy back up and the two save Maggie before fleeing from the house.
Fifteen 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 figure with a scarred wrist(Garrett) is seen holding a garden trowel.