Gossip thrives amid the Mercedes-Benzes, mega mansions, and perfectly manicured hedges in the exclusive town of Rosewood, Pennsylvania. Behind their big Gucci sunglasses, beneath their perfectly pressed Polos, everyone has something to hide, especially high school juniors Spencer, Aria, Emily, and Hanna. Spencer covets her sister's gorgeous new boyfriend. Aria is having an affair with her English teacher. Emily is infatuated with the new girl at school. And Hanna is using some ugly tricks to stay beautiful. Deeper and darker still is a horrible secret the girls have shared since sixth grade—a secret they thought was safe forever.

Then the girls start receiving threatening messages signed by someone named "A." Could "A" be Alison, the ultra-popular girl who disappeared three years earlier? Alison was their best friend. She knew everything about them. But if Alison isn't "A," who is? And how could anyone else know so much about the bad girls they were and the bad girls they've become? Suddenly, their secrets—the big ones, the little ones, even the long-buried ones—no longer seem so secret. Suddenly, nothing is safe in Rosewood.