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.

Spencer stole her sister's boyfriend. Aria is brokenhearted over her English teacher. Emily likes her new friend Maya . . . as much more than a friend. And Hanna's obsession with looking flawless is literally making her sick. But the most horrible secret of all is something so scandalous that the truth would ruin them forever.

The stalker's identity has finally been revealed but high school juniors Spencer, Emily, Hanna, and Aria seem condemned to repeat the past as they resume their unrepentant ways.

Emily's been shipped off to Iowa to live with her überconservative cousins. Aria's boyfriend is behind bars--because of her. Spencer's afraid she was involved in Ali's murder. But Hanna's fate is far worse: she's clinging to life in the hospital because she knew too much. If these girls don't start listening to me, Hanna's going to look like the lucky one.