Following the tragic deaths of his wife and son, antiques dealer Terence Cave is left alone to raise his only daughter, fifteen-year-old Bryony. As the young girl struggles to come to terms with the loss of her twin brother, Reuben, Terence observes changes in her behavior--changes he believes are more dangerous than the common trials of adolescence. In order to protect her, he begins to follow her whenever she leaves the house, becoming increasingly paranoid about her safety.