High school seniors Aria, Emily, Spencer, and Hanna barely survived their most recent encounter with A. The police don't believe that the real A is still out there, but the girls know what and who they saw. If they don't track down this final tormentor soon, A will silence them forever.

When twenty-one-year-old social media star Chelsea Dawson disappears, Seneca, Maddox, Aerin, and Madison, noticing that she looks exactly like Aerin's murdered sister, Helena, investigate, determined to find the connection.

Four teens who met on an online cold-case solving forum attempt to solve another teen's murder--

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.

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.

High school seniors Aria, Emily, Hanna, and Spencer have all have their secrets. And their stalker 'A' knows everything. A has used their secrets to torture the girls, causing them to lose boyfriends, get kicked out of school, put into jail, and constantly hounded by the press. The Girls are determined to stop A once and for all. Because everybody knows the only way for two people to keep a secret is if one of them is dead.