When their best friend mysteriously vanishes, four girls believe their secrets are safe forever. But when they begin to receive threatening messages, the truth may be only one text away. Set one year after the disappearance of Alison (series star Sasha Pieterse), the manipulative and vindictive queen bee of their group, the series revolves around four 16-year-old girlfriends — Aria (series star Lucy Hale), Spencer (series star Troian Bellisario), Hanna (series star Ashley Benson), and Emily (series star Shay Mitchell) — who have lost touch with one another until each begins to receive mysterious messages from "A," suggesting that Alison is watching them. Linked by their former bond and the panic and confusion the enigmatic messages cause, the friends are suddenly reunited, though each girl has her own secrets — secrets she wants no one else to know.

"The past will come back to haunt you" — this saying has never been more true than for Aria, Spencer, Hanna and Emily. As the girls try to pay tribute to Alison's memory, the four friends are faced with unresolved issues. Aria has an unsettling encounter with Byron's old mistress, Meredith (recurring guest star Amber Borycki – The Big Bang Theory, Harper's Island). Emily is forced to see Toby Cavanaugh (recurring guest star Keegan Allen) in a new light. Spencer is reeling in the wake of someone with a vengeance and Hanna is desperate to have her boyfriend forget the "old" Hanna. With "A" taunting them every step of the way, can the girls forge ahead and leave the past behind? Meanwhile, Detective Wilden (recurring guest star Bryce Johnson – Popular) uses his new connection to Hanna and Ashley to his benefit. But will he get the answers he's looking for by making himself at home in the Marin household?

Tired of being harassed by "A," the girls decide to put a stop to the anonymous emails and text messages by blocking all unknown users. But just because the mysterious ghost isn't looming over their heads, the girls' problems are far from over. With a surprise visit by a father, a surprise gift from a prospective paramour, getting some unwanted advice and an unwanted scholarly accolade — the girls' lives continue to get more complicated by the day. Do the four girls really think they can keep "A" away forever?

Trying to ignore "A" backfired for the girls and now they must pay the penalty. Whether it is a family in crisis, parental praise for an undeserved honor looming over their head or wearing totally unfashionable scrubs while working off a debt — the girls must deal with it all. And on top of that the lipstick message from "A" is still haunting them. Meanwhile, the girls must also deal with typical teenage drama — the boy kind. A new guy arrives and pushes the status quo, a secret love is tested, a new romance blooms and an old romance floats in the grey area.

The Homecoming decorating committee is knee-deep in balloons and girls are jockeying for position to be the queen, while four friends from Rosewood just try to stay afloat. One girl makes a shocking choice for a homecoming that puts her in a possibly sticky situation. Will the girls be able to protect one of their own or will the evening turn out to be a night they can never forget?