Olive Penderghast (EMMA STONE) is a 17-year-old high school student who believes she's invisible to most of her fellow students despite the support of her parents, Dill (STANLEY TUCCI) and Rosemary (PATRICIA CLARKSON), and English teacher Mr. Griffith (THOMAS HADEN CHURCH) who's teaching his class "The Scarlett Letter."
Her fortunes change when ultra-religious Marianne (AMANDA BYNES) overhears Olive tell a white lie to her best friend, Rhiannon (ALY MICHALKA), about losing her virginity. Olive only told her that to stop Rhiannon's constant nagging about Olive's weekend, but it doesn't take long for Marianne to tell her boyfriend, Micah (CAM GIGANDET), and others, with the scandalous tale quickly spreading to the entire student body.
While school mascot Todd (PENN BADGLEY) doesn't pay any heed to the gossip, Olive's newfound notoriety soon has her meeting both guidance counselor Mrs. Griffith (LISA KUDROW) and Principal Gibbons (MALCOLM McDOWELL), the latter of whom puts her into detention for some salty language. It's there that she meets Brandon (DAN BYRD) who wants to stop the torment of being a closeted gay student and thus asks Olive to lie that she slept with him to fool others into thinking he's straight.
She reluctantly does, but that faux encounter and the subsequent rumors that quickly spread soon have her in the spotlight of various students. Among them are other guys who similarly want to benefit from her reputation, while Marianne and her followers want her kicked out of school. Realizing she can't beat the gossip that turns into other rumors, Olive decides to play up the lifestyle of a floozy, including proudly wearing her own letter "A" on her clothed chest.
From that point on, and as her friendship with Rhiannon takes a turn for the worst, Olive must contend with that, many of the guys wanting their name and reputation associated with her, and other developments that come along with her self-perpetuating status.