On a winter night on a remote Nebraska road, twenty-seven-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 impostor. Shattered by her brother's refusal to recognize her, Karin contacts the cognitive neurologist Gerald Weber, famous for his case histories describing brain disorders. Weber recognized Mark's condition as a rare case of Capgras syndrome — the delusion that people in one's life are doubles or impostors — 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.