Returning home from a trip to Paris, Boston medical examiner Maura Isles confronts a nightmarish scene. Police cruisers line her street and slumped in a car at the end of Maura's driveway is a dead woman, her head bloodied by a gunshot wound. Cops and neighbors gape at her as she approaches. Only when Maura looks at the victim does she understand why everyone is so shocked by her arrival. The dead woman looks exactly like Maura. Within a week, the DNA lab delivers a stunning verdict: the dead woman is Maura's twin. A twin she never knew she had. Detective Jane Rizzoli, now eight months pregnant, tackles the case as chief investigator. But for Maura, this murder is far more personal and frightening.