When the coroner, Dr. When Maura Isles returns home in Boston at night from a trip to Paris, an unpleasant surprise awaits her: her house is surrounded by police officers. At the same time, Isle's colleagues are shocked at her arrival, as they had already assumed she was dead. With good reason, because a woman who was found murdered near her house looks confusingly like her. That evening, Maura's Isles life changes radically. Since she was adopted at an early age, she knows nothing about her real origins. Now the murder victim turns out to be Maura's twin sister, who was apparently looking for the family history. Maura continues her sister's search and tracks down her biological mother in a psychiatric detention center. While the police, led by the heavily pregnant detective Jane Rizzoli, are looking for a dangerous serial killer, an unspoken threat hovers over the confused Maura alongside the painful question of her origins: who killed her sister and why?