When one of their tightly-knit group mysteriously disappears, four high school girls find their friendship difficult to maintain when they begin receiving taunting messages from someone who seems to know everything about their past and present secrets.

Best friends Spencer, Aria, Emily, and Hanna find themselves under suspicion when the mysterious stranger named A tries to blame one of the girls for the death of their friend Alison and threatens to reveal the girls' darkest secrets.

Former best friends Hanna, Aria, Spencer, and Emily, who lost their best friend and leader, Alison, years ago when she disappeared, find themselves in grave danger when they pursue the truth about Alison.

Deciding to come clean about a terrible mistake they committed during spring break, high school seniors Aria, Emily, Hanna, and Spencer hope to end the blackmail of a ruthless stalker who retaliates by plotting a murderous revenge.

After seeing a girl who looks just like herself in a violent video, Emma, who lives with a foster family, begins a search to find out what happened to the girl.

Seventeen-year-old Sutton Mercer watches from the afterlife as her long-lost twin sister, Emma Paxton, pretends to be her in order to identify Sutton's killer.