When the body of four-year-old John Vincent is discovered, hidden in a midden, the already unhappy Cornwall family begins to unravel. With no obvious reason for his murder, a scapegoat must be found and his elder half-sister Charlotte, the sleepwalker, neatly fits the bill. After facing the horror of a trial by jury, Charlotte is found not guilty but suspicion still follows her as she takes up almost unpaid work as a teacher in the untrustworthy Mrs. Armitage's school. After her fellow teacher and friend, Miss Lamb, vanishes, followed by the tragic death of one of Charlotte's much-loved pupils, the finger of blame is pointed again. Now even Charlotte begins to believe that she is losing her mind and may well be a murderess.