Heartless is a novel that is part of the Pretty Little Liars series by the author Sara Shepard. The story follows the four main characters of the book, Emily, Hanna, Spencer, and Aria, as they deal with the aftermath of the disappearance and death of their best friend, Alison. After a slumber party, Alison leaves the barn where the girls are sleeping on Spencer's property, and she is never seen or heard from again. Each of the four remaining girls starts receiving text messages that appear to be from their missing friend. Then, Alison's body is found in the hole the construction workers dug and were pouring cement in to build the family gazebo. The anonymous texter turns out to be Hanna's ex-friend Mona, who tries to run Hanna over with her SUV.

A new text messenger arrivers on the scene, using the same signature of "A" and claiming she has information on Alison's death and true killers. Ian, who was blamed for the murder is on the run, and he too is communicating with the girls via instant messenger (IM) to help them find the real killer. A's text messages send Hanna to a mental hospital, Emily to Dutch Amish country, Spencer to believe her mother killed Ali and that her father was having an affair with Ali's mother, and Aria to believe Ali killed herself.

By the end of the novel, the girls even find themselves under arrest for their best friend's murder. The girls are released when the police find evidence on a construction worker's computer that he is the anonymous A. The police also find another body of a girl where he was doing some plumbing work.