As the story opens, Sutton Mercer is lying in a bathtub in an unfamiliar room. She can't remember how she came to be there but she also can't remember much of anything about her life. Then a girl enters the bathroom and Sutton tries to make her presence known. The girl completely ignores her and it takes only a little longer for Sutton to realize she's dead. The girl in the bathroom is Emma Paxton. Emma and Sutton are identical twins and were separated, apparently at birth or as very young children. Sutton was adopted by an affluent family while Emma remained with their mother. Sutton has apparently led the life of an indulged child and has turned out to be a selfish teenager. Emma lived with her mother, who showed signs of manic depression, until she was about five, and has been shuffled from one foster home to another.

Emma is in a foster home when the son of her foster parent shows his mother a "snuff film" featuring Sutton, though it appears to be Emma. Emma does some research and discovers Sutton's identity. She finds her sister on Facebook and leaves a message with the suggestion they might be twins. Emma receives a reply a short time later with a meeting place and time. She assumes the message is from Sutton. Sutton knows that isn't the case but is unable to share that information with Emma. Emma is also warned not to tell anyone of her existence, warning that it might be dangerous. She shows up on time but Sutton never appears. Emma is suddenly confronted by several of Sutton's friends who are on their way to a party. Emma, recalling the message that revealing her identity could be dangerous, decides to go along with the situation until she can meet with Sutton in person. She concludes her evening at a party with Sutton's friends and is dropped off at Sutton's house. She locates Sutton's bedroom and falls asleep. The next morning, she tries to explain the situation, first to Sutton's parents and then to the police, but discovers that Sutton has been so volatile and wild that no one believes her.

Then she receives a note that indicates Sutton is dead and that she'll be killed if she doesn't continue to play along. Feeling she has no other option, she decides to look for Sutton's killer. Meanwhile, Sutton is tied to Emma and has begun to have a few memories. Both Emma and Sutton come to realize that Sutton was not an angel and that she'd hurt a lot of people with her reckless pranks and selfishness. Emma comes to believe Sutton's sister Laurel may be Sutton's murderer but the evidence isn't complete. Emma decides she should run away but a teenager named Ethan Landry becomes the only one to realize that Emma isn't Sutton. Ethan pledges to help Emma solve Sutton's murder and she returns to Sutton's home with that in mind.