The narrative begins with a family document detailing the violent looting of a sacred yellow diamond, known as the Moonstone, from an Indian temple, cursed to bring ruin upon anyone who seizes it. Fifty years later, the diamond is bequeathed to Rachel Verinder by her disreputable uncle on her eighteenth birthday, brought to the Yorkshire estate by her cousin Franklin Blake. On the very night of the celebration, the magnificent gem vanishes from her cabinet without a trace. A local police superintendent bungles the initial investigation by antagonizing the household staff and failing to secure the premises, leading to the engagement of the legendary London detective, Sergeant Cuff.
Cuff immediately identifies a crucial clue in a paint-smear on the doorway and deduces that the theft involves someone inside the household, casting suspicion on a reformed housemaid named Rosanna Spearman, who harbours a secret tragedy and an unrequited devotion to Franklin Blake. Meanwhile, Rachel Verinder behaves with intense agitation, abruptly refusing to allow any search of her wardrobe, announcing her sudden departure, and exhibiting an unaccountable hostility toward Franklin Blake. Sergeant Cuff steadily tightens his investigation, convinced that the mystery of the missing diamond is intimately connected to Rachel's own strange conduct. As the household plunges deeper into distrust and anxiety, the arrival of further complications—including suspicious Indian wanderers and conflicting witness accounts—sets the stage for a profound psychological and forensic revelation.
