Following the shock of the Moonstone's disappearance, the celebrated detective Sergeant Cuff is called to Yorkshire by Franklin. Rachel is devastated by the loss of the diamond and, despite Franklin begging her to stay, leaves for London, casting suspicion on herself. Cuff focuses on the odd behaviour of Rosanna, a reformed thief who now works as a servant in the Verinder house. Before he has chance to talk to her, she disappears. Betteredge and Cuff rush to the windswept beach where she often walks with the hope of saving her and finding out the truth.