The small resort of Tranquillity, Maine, seems like the perfect spot for Dr Claire Elliot to shelter her adolescent son, Noah, from the temptations of the big city and the traumatic 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 an outbreak of teenage violence, far more deadly than anything she has encountered in the city, erupts in the local school.

In trying 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. Terrified that Noah too is at risk, she must prove her theory before everything she loves is destroyed.