Colonel Herncastle, an unpleasant former soldier, brings the Moonstone back with him from India where he acquired it by theft and murder during the Siege of Seringapatam. Angry at his family, who shun him, he leaves it in his will as a birthday gift to his niece Rachel, thus exposing her to attack by the stone's hereditary guardians, who will stop at nothing to retrieve it.
Rachel wears the stone to her birthday party, but that night it disappears from her room. Suspicion falls on three Indian jugglers who have been near the house; on Rosanna Spearman, a maidservant who begins to act oddly and who then drowns herself in a local quicksand; and on Rachel herself, who also behaves suspiciously and is suddenly furious with Franklin Blake, with whom she has previously appeared to be enamoured, when he directs attempts to find it. Despite the efforts of Sergeant Cuff, a renowned detective, the house party ends with the mystery unsolved, and the protagonists disperse.
