Everyone has something to hide—especially high school juniors Spencer, Aria, Emily, and Hanna. Spencer covets her sister's boyfriend. Aria's fantasizing about her English teacher. Emily's crushing on the new girl at school. Hanna uses some ugly tricks to stay beautiful. But they've all kept an even bigger secret since their friend Alison vanished. How do I know? Because I know everything about the bad girls they were, the naughty girls they are, and all the dirty secrets they've kept. And guess what? I'm telling.