Returning home from Paris, Boston medical examiner Maura Isles is greeted by a nightmarish scene. Slumped in a car in her driveway is a dead woman, a gunshot wound to her head. Cops and neighbours stare at her as she approaches; and when Maura looks into the car, she understands why. The body on the front seat is her mirror image. In the autopsy that follows, Maura discovers more and more parallels between her and the dead woman, right down to their identical blood group. She is even more confused when her friend, Detective Jane Rizzoli, learns that, according to all available documentation, the woman did not even exist until two years previously.

Who was this woman? What was her relationship with Maura? And who is the killer's real target? Maura, who was given up for adoption at birth, knows she must confront the truth about her own mysterious origins. As she tracks down the dead woman's identity, she uncovers a shocking revelation about the mother she never knew. Drawn into a dangerous and manipulative game, Maura and Rizzoli are led ever deeper down a path of dark untruths and murderous deceit.