1 year ago, Allison, the queen bee of the four friends, Aria, Spencer, Hanna, and Emily, went missing. Since her disappearance, the four girls became less close, with Aria moving to Iceland, Hanna losing weight, Spencer in competition with her older sister, Melissa, and Emily experimenting with her sexuality. When Aria comes back from Iceland, she hooks up with a guy from a bar, who turns out to be her English teacher. Hanna gets arrested for stealing sunglasses from the mall, and her mother has sex with the arresting officer to get Hanna out of trouble.
Tired of always coming second to Melissa, Spencer makes out with her fiancee, causing the engagement to end. Emily meets the new girl who moves into Rosewood, Maya, and feels something a little more than friendship for her. When Aria moves back to Rosewood, all four girls receive strange and threatening messages from "A", who they believe could possibly be Allison, considering the messages were about exposing both present and past secrets- secrets only Allison knew about. The past secrets included Aria's father cheating on her mother with his student, Spencer kissing Melissa's previous boyfriend, Emily's romantic feelings for Allison, Hanna's bulimia to become skinny, and one night when Allison claimed to see a boy named Toby peeping through the window and threw a stink bomb in the garage, causing a fire, and blinding Toby's step-sister, Jenna. Even after Allison's dead body was found and a funeral was held, the girls keep receiving texts from A.
Aria, Spencer, Hanna, and Emily must find out who A is before he/she reveals all of their deepest, darkest secrets..