Boston medical examiner Dr. Maura Isles is shocked to discover that the murdered woman looks exactly like her. For Maura, an only child, a DNA test confirms the startling fact: the mysterious doppelgänger is in fact her twin sister. Now an already bizarre homicide investigation becomes a disturbing excursion into a past full of dark secrets and twisted truths. It is a journey that leads Maura to the mother she never knew-an icy and cunning woman who gave Maura life... and who just might have a plan to take it away.