Rule number one of being an effective stalker: get to know your prey, the trouble they get into, the messes they make, and the secrets they keep. Hanna's desperate for a very personal session with her gym trainer. Emily is number one on Santa's naughty list. Aria's old flame from Iceland is about to land her in hot water. And Spencer's resorting to some backhanded tactics to get what she wants.

Aria, Emily, Hanna, and Spencer's stalker, A, wants to do more than ruin their lives, he wants to end them.

Struggling to return to normalcy after surviving Alison's attempt to kill them, Spencer, Aria, Emily and Hanna endure a series of disasters before realizing that they are being stalked by a new "A" who knows all of their secrets.

Even though the disappearance of Alison DiLaurentis has been solved, Spencer, Emily, Hanna, and Aria harbor other secrets which could interfere with their perfect lives, as they try to forget what happened to them in Jamaica last spring.