When the famous Indian Moonstone disappears at Rachel Verinder's 18th birthday, Franklin Blake seeks Gabriel Betteredge's help to find the jewel.

Detective Sergeant Cuff heads to Yorkshire in the wake of the shocking disappearance of the Moonstone. The loss of the diamond devastates Rachel and she leaves for London as Franklin begs her to stay.

Rachel's cousin Miss Clack talks to Franklin after arriving at the Verinder house, explaining what happened when Lady Verinder and Rachel reunited in London. She also implicates another of Rachel's cousins.

With a letter from Rosanna in hand, Sergeant Cuff arrives at Franklin's lodgings, while the Verinder's solicitor Mr Bruff learns that Rachel has accepted a proposal from Godfrey.

Franklin returns to the Verinder house in Yorkshire after a tearful meeting with Rachel. He seeks answers and Dr Ezra Jennings offers to help him. Franklin feels desperate as he tries to win back Rachel and find the thief.