Boston medical examiner Maura Isles is no stranger to the sight of dead people. However, this time she is overcome by real horror. The woman found murdered in a car near Maura's house is exactly like her. It soon turns out that the murdered woman is her twin sister, whose existence Maura, adopted in infancy, did not even suspect. But whose cruel will divided the sisters? And who really was the killer's target? The investigation of this unusual crime leads into the distant past, full of dark secrets and incredible discoveries...