(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
The Moonstone is a magnificent yellow diamond 'large as a plover's egg' which had originally adorned the forehead of the Hindu moon-god. It was subsequently looted at the siege of Seringapatam in southern India in 1799 by Colonel John Herncastle. On his return to England he was ostracized by his family and society, and in revenge for a slight by his sister, Lady Julia Verinder, he leaves the diamond, said to carry a curse, to his niece Rachel Verinder. Rachel's cousin, Franklin Blake, is to deliver the diamond to the Verinder house near Frizinghall on the Yorkshire coast.
The Moonstone is presented to Rachel at a dinner-party for her eighteenth birthday. The guests include Godfrey Ablewhite, another cousin; Mr Candy, the family doctor; Mr Murthwaite, a celebrated traveller in India; and Drusilla Clack, an interfering evangelist. The party goes badly. Rachel and Franklin Blake have become fond of each other while decorating her sitting-room door and Rachel had earlier refused a marriage proposal from Ablewhite. In addition, Blake quarrels with Mr Candy about the competence of doctors.
Blake had been followed in London and Murthwaite identifies three Indians seen near the house as high-caste Brahmins. Rachel places the diamond in her bedroom cabinet but the next morning it is missing.
The local police superintendent, Seegrave, is a bungling incompetent so Blake calls in the celebrated Sergeant Cuff of the detective police. He rules out the suspicious Indians but realizes the importance of smeared paint on Rachel's sitting-room door. The smear has been made by an article of dress, whose owner is almost certainly the thief. Rachel behaves inexplicably, obstructing the investigation and refusing to have anything more to do with Franklin Blake. Cuff concludes that she has stolen her own diamond assisted by Rosanna Spearman, a deformed housemaid fascinated by the local quicksand. Rosanna is a reformed thief who is acquainted with a dubious London moneylender, Septimus Luker. She is also in love with Franklin Blake and after acting strangely drowns herself in the Shivering Sand. Cuff is dismissed from the case by Lady Verinder but correctly predicts future developments.
In London, both Ablewhite and Luker are attacked and searched, Luker losing a receipt for a great valuable. Lady Verinder dies of a heart condition and Rachel reluctantly agrees to marry Ablewhite whose father has become her guardian. They move to Brighton where they are visited by Mr Bruff, the family solicitor. The engagement is broken off when he reveals that Ablewhite is in debt and is marrying Rachel for her money.
Blake returns from travels abroad but Rachel refuses to see him. Determined to restore her good opinion, he revisits Yorkshire where Rosanna Spearman's only friend, Limping Lucy, gives him a letter from the dead housemaid. This leads him to the Shivering Sand where Rosanna has hidden his nightgown, smeared with paint, with a confession that she concealed the nightgown and killed herself out of love. The confused Blake returns to London and contrives a meeting with Rachel at Mr Bruff's house in Hampstead. There she tells him that she knows he had financial problems and with her own eyes saw him take the diamond. Her own actions have been to protect his reputation.
Blake meets Mr Candy's assistant, Ezra Jennings, who saved Candy's life from a fever caught after the birthday dinner. Jennings had recorded Candy's delirium which revealed that Candy had secretly given Blake opium to prove his point in their argument. Blake therefore unknowingly 'stole' the diamond under the influence of the drug, in order to keep it safe. Jennings explains to Blake that if he takes opium again under similar conditions he may repeat his actions of the previous year and reveal where he placed the diamond. Blake agrees and the experiment is conducted with Mr Bruff as an observer. Blake takes a substitute gem but fails to reveal the Moonstone's hiding place. Rachel, really in love with him, is also present and has already forgiven him.
Bruff in the mean time has Luker's bank watched. The moneylender is observed passing the diamond to a sailor who is followed to a dockside inn. Later the same night he is murdered. Cuff, brought out of retirement by Blake, discovers that the sailor is Godfrey Ablewhite in disguise. He was the real thief and stole the gem to save himself from financial ruin. He has been killed by the Indians who have now recovered the diamond. In a religious ceremony witnessed in India by Murthwaite, the Brahmins return the diamond to the god of the moon.