Transferring to a new high school, freshman Charlotte "Charlie" Healey faces tough choices as she tries to shed her "mean girl" image.

After high school juniors Spencer, Emily, Hanna, and Aria think they have seen Ali, their missing and presumed dead friend, they pursue clues from a mysterious stalker, trying to find out if she is still alive.

Spencer, Hanna, Aria, and Emily, friends since seventh grade at the exclusive Rosewood school in Pennsylvania, find their history of telling small, convenient little lies catching up with them when a mysterious person named A begins making threats, hinting that he or she knows what really happened to their murdered friend Alison.

Fifteen-year-old Essie Green, an orphan who has been raised by her secular Jewish grandparents in Michigan, experiences conflicting loyalties and confusing emotions when her aunt, uncle, and cousin move back from New York, and her very religious cousin tries to fit in with the other football players at Essie's high school, one of whom is Essie's popular new boyfriend.