Predicated on the Pretty Little Liars series of young adult books by Sara Shepard, the show follows the lives of 4 girls -- Emily, Hanna, Aria, and Spencer -- whose clique drops after these queenbee, Alison's disappearance. 1 year after they begin receiving messages from someone using the name"A" who threatens to expose their secrets -- for example long-hidden ones that they thought just Alison knew.