The Echo Maker

On a winter night on a remote Nebraska road, 27-year-old Mark Schluter flips his truck in a near-fatal accident. His older sister Karin, his only near kin, returns reluctantly to their hometown to nurse Mark back from a traumatic head injury. But when he emerges from a protracted coma, Mark believes that this woman–who looks, acts, and sounds just like his sister–is really an identical impostor. Shattered by her brother's refusal to recognize her, Karin contacts the cognitive neurologist Gerald Weber, famous for his case histories describing the infinitely bizarre worlds of brain disorder.

Weber recognizes Mark as a rare case of Capgras Syndrome, a doubling delusion, and eagerly investigates. What he discovers in Mark slowly undermines even his own sense of being. Meanwhile, Mark, armed only with a note left by an anonymous witness, attempts to learn what happened the night of his inexplicable accident. The truth of that evening will change the lives of all three beyond recognition.

Synopsis from Open Library.