After their friend Alison, missing for more than three years, turns up dead, four former best friends, each of whom had a secret known only by Alison, begin receiving frightening messages from someone identified by the letter "A."

The four former best friends try to overcome their friend Ali's murder, but become alarmed when they receive text messages from the mysterious "A" and learn that their friend's accused killer has been released from jail on bail.

Former best friends Emily, Aria, Hanna, and Spencer continue to live in fear as they struggle to keep hidden even more secrets and someone new begins stalking them via their cell phones.

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.

"It's spring break, and the pretty little liars are trading in Rosewood for a cruise vacation. They want nothing more than to sail into the tropical sunset and leave their troubles behind for one blissful week. But where Emily, Aria, Spencer, and Hanna go, A goes, too. From scuba diving to tanning on the upper deck, A is there, soaking up all their new secrets. Emily is smooching a stowaway. Aria's treasure-hunting partner is a little too interested...

Aria, Emily, Hanna, and Spencer find their lives unraveling when A's plans change and the girls are taken to the police for questioning.

Now seniors in high school, Spencer, Emily, Hanna, and Aria's friendship has been torn apart by new lies and stress, but they are drawn back together when anonymous messages threatening to reveal their secrets resume.

Hanna, Spencer, Emily and Aria are trying to move on and forget about their old friend Alison DiLaurentis and all the torture she put them through, but someone is trying to keep her spirit alive.