Twins Regina and Renata are so identical that even their mother can't tell them apart. Eerily attuned to one another, they often speak together in a secret language no one else can understand. Then tragedy strikes when one of the twins is viciously murdered. The remaining twin is so traumatized, she reverts to the cryptic language she once shared with her sister. What's more, since identical twins share identical DNA, there is no way to tell which girl was the victim. The parents believe the surviving twin is Renata and have no choice but to commit her to an institution. There she lives, incomprehensible to the rest of the world. Until the day Mark, a family friend, comes to visit and the young girl utters her first intelligible word.

From that day forward Mark spends hours with Renata at the hospital. When she finally begins to communicate again in English, she reveals virtually no memory of the past. Mark remains a strong link between Renata and her former life -- a fact that drives him to a resolute sense of responsibility for the fragile girl. Brilliant and beautiful, Renata gradually reacclimates herself to the outside world. But she is far from recovered. Her nightmares grow steadily more frightful, followed by wild fits of hysteria and dark mood swings.

Meanwhile, as Seattle is plagued by a grisly serial killer, Mark has a terrifying realization: the murders coincide with Renata's strange outbursts. Could she be the killer? Determined to dispel his suspicion, he stakes out Renata's home. But his nocturnal vigils do nothing to ease his fears. For the unholy sight he witnesses on one of Seattle's most bone-chilling winter nights will haunt his soul for the rest of his life....