Aria; Emily; Spencer; Hanna and Alison have been best friends since the third grade. They go everywhere together; thinking no-one can come between them. If anyone is the ringleader of the group it is Alison; and the other girls cannot help but confide all their secrets to her. One night; during a sleepover; Alison goes missing. Her body is never found. The girls mourn her death but move apart after time; assuming their secrets have disappeared with Alison too. Three years later and Aria is having an affair with her teacher; Emily is questioning her sexuality; Hanna is a thief; and Spencer is flirting with her sister's fiance. They all think their secrets are safe; until they starting receiving messages from the mysterious A - who knows exactly what they are all up to; and is threatening to spill the beans . . .