Four friends - Emily, Spencer, Aria and Hanna - are keeping a secret that could ruin their flawless image. And someone named "E" threatens to expose it. Could this be their friend Alison, who disappeared three years ago? Or is it Melissa, Spencer's older sister, whose dream is to eliminate her competition? What about Toby, the mysterious neighbor who left town immediately after Alison disappeared? One thing is certain: any crumpled note, strange SMS message, threatening letter - and the girls lose everything.