A first-year resident at a major Boston children's hospital, Peyton Shields finally has the career she's always dreamed of—even though her marriage to Kevin, an up-and-coming young lawyer, is suffering from her hard work and impossibly long hours. But a late-night drive home in a heavy snowstorm changes everything when she is forced off the road.

No one, not even Kevin, believes Peyton's claims that the "accident" was deliberate. Suddenly, her wonderful life has turned dark and uncertain, and the terror has only just begun. Her husband is growing inexplicably distant and bitter, accusing her of paranoia, betrayal, and infidelity. And a series of bizarre and frightening events is moving Peyton steadily closer to a faceless, resourceful enemy who is watching her every move.