When twenty-seven-year-old Mark Schluter gets into a car accident, his older sister Karin returns to her hometown against her will to take care of her brother. But when he comes out of his coma, he believes that she has been replaced by a completely identical doppelganger, who may wish him harm, even though he looks, acts and speaks like his own sister. Desperate, Karin turns for help to a recognized neurologist, a real star of medicine, Gerald Weber. He arrives, seizing the opportunity to study a rare case of progressive Capgras syndrome, an infrequently occurring delusional disorder. But everything turns out to be even more complicated, and constant communication with the patient begins to influence the mind of the doctor himself. Mark, armed only with a strange note left in his room by an unknown person, is trying to find out what happened on the night of the car crash. The truth about her will irreversibly change the lives of all three of them.