The small resort town of Tranquillity, Maine, seems like the perfect sport for Dr Claire Elliot to shelter her adolescent son, Noah, from the temptations of the big city and the lingering memory of his father's death. Claire's hopeful that she can earn the trust of the town as she builds a new practice. But all her plans unravel with the onset of winter, when a rash of teenage violence, far more deadly than anything she's encountered in the city, erupts in the local school.
As she tries to find a medical explanation for this murderous epidemic, Claire stumbles upon an insidious evil which has blighted the town's past and threatens its future. Fearful that Noah too is at risk, she must race to prove her theory before everything she loves is destroyed.