It's the last year of college for a tight knit group of sisters in the Theta Pi sorority. While party-girl Chugs (MARGO HARSHMAN) might be the epitome of the wild bashes they throw, Jessica (LEAH PIPES) is the snarky ruler of the bunch who has her eyes set on a future with Kyle (MATT LANTER), the son of an influential senator.
Among the other seniors is Cassidy (BRIANA EVIGAN), whose boyfriend Andy (JULIAN MORRIS) is the brightest in the school. Her best friend, the bookish Ellie (RUMER WILLIS), doesn't have a boyfriend, but Claire (JAMIE CHUNG) is having fun with Mickey (MAXX HENNARD), despite his wandering eyes. And then there's Megan (AUDRINA PATRIDGE) who's dating Chugs' brother, Garret (MATT O'LEARY), but isn't happy he's recently cheated on her.
Accordingly, she and a few of the sisters concoct a prank where they'll convince Garret to slip her some date rape drug to take advantage of her, and then fake her death including the disposal of her body in a remote location. When they get there, he freaks out and panics when he hears the body won't sink due to air in the lungs, and thus plunges a tire iron deep into her chest.
It's only then that he realizes he was duped, but it's too late as she's now dead for real. Knowing that going to the police will ruin all of their lives, Jessica convinces the others -- including the reluctant Cassidy -- that they must abide by one of their sorority's tenets -- secrecy -- and not tell anyone what happened or that they dumped Megan's body down an abandoned mine shaft.
Eight months later, the girls find themselves graduates and return to the sorority for one last wild night where housemother Mrs. Crenshaw (CARRIE FISHER) turns the house over to them. But upon the arrival of Megan's younger sister, Maggie (CAROLINE D'AMORE), who not only looks like her sibling but is also going to pledge her sorority next year, things start getting weird, increasingly creepy, and then downright deadly.
And that's because a killer cloaked in a black graduation gown has arrived and starts to pick them off one by one, with various clues (text messages, photos, and a modified tire iron as the weapon of choice) leaving the survivors in a panic wondering if the assailant might be Megan having somehow survived that night months ago or perhaps has returned from the dead for revenge.