Sixteen-year-old Cara Lange has been a loner ever since she moved away from her best and only friend, Zoe, years ago. She eats lunch with the other girls from the track team, but they're not really her friends. Mostly she spends her time watching Ethan Gray from a distance, wishing he would finally notice her, and avoiding the popular girls who call her "Choker" after a humiliating incident in the cafeteria.
Then one day Cara comes home to find Zoe waiting for her. Zoe's on the run from problems at home, and Cara agrees to help her hide. With her best friend back, Cara's life changes overnight. Zoe gives her a new look and new confidence, and next thing she knows, she's getting invited to parties and flirting with Ethan. Best of all, she has her BFF there to confide in.
But just as quickly as Cara's life came together, it starts to unravel. A girl goes missing in her town, and everyone is a suspect—including Ethan. Worse still, Zoe starts behaving strangely, and Cara begins to wonder what exactly her friend does all day when she's at school. You're supposed to trust your best friend no matter what, but what if she turns into a total stranger?