Alison "Ali" DiLaurentis was once the center of the main girls' group, but disappeared one night at a group sleepover. A year after her dissapearance, her body was found by construction workers behind her former house. No one is sure what happened to Alison that night. The mysterious "A" who sends Aria, Emily, Hanna, and Spencer chilling messages is thought to be connected to her in some way due to the fact that "A" knows things about the girls that only Alison was thought to know.