Margot Lewis is a teacher at an exclusive high school in the English university town of Cambridge. In her spare time, she writes an advice column, “Dear Amy”, for the local newspaper. When one of Margot’s students, fifteen-year-old Katie, disappears, the school and the town fear the worst. And then Margot gets a “Dear Amy” letter unlike any of the ones she’s received before. It’s a desperate plea for rescue from a girl who says she is being held captive and in terrible danger—a girl called Bethan Avery, who was abducted from the local area twenty years ago…and never found. The letter matches a sample of Bethan’s handwriting that the police have kept on file since she vanished, and this shocking development in an infamous cold case catches the attention of criminologist Martin Forrester, who has been trying to find out what happened to her all those years ago. Spurred on by her concern for both Katie and the mysterious Bethan, Margot sets out—with Martin’s help—to discover if the two cases are connected. But then Margot herself becomes a target.