Pretty little liars

It has been one year since Alison, then the queen bee of the bunch, went missing. Spencer, Aria, Adam, Hanna and Emily have gone on with their lives, though they've grown apart. Now in high school, each of them finds themselves facing a new set of challenges when anonymous text messages from 'A' threaten to expose all of their secrets - both old secrets the teens were sure only Alison knew, and newer secrets developed after Alison's disappearance. Who is 'A' and how does he or she know the things he or she knows?