In the exclusive Philadelphia suburb of Rosewood, Alison is the Queen Bee of her elite seventh grade hive. BFs Aria, Hanna, Spencer, and Emily vie for her attention, even as each of them hides a hideous secret only Alison knows. So when Alison goes missing after a slumber party, never to be seen again, each girl is heartbroken, but also a little relieved. Now it is three years later, and though the four girls have grown apart, they are each still hiding something. Artsy Aria is carrying on an affair with one of her teachers, fashionista Hanna shoplifts to accessorize her trendy outfits, blue-blood Spencer is sleeping with her older sister’s boyfriend, while straight-A Emily is trying to ignore her attraction to a new female classmate. When the girls begin receiving threatening text messages and emails that from someone known only as "A," they must confront the fact that against all odds, it appears Alison is back. Could Alison still be alive? And if so, why is she so determined to uncover all their dirty little secrets?