Moonstone – a huge yellow diamond – from time immemorial decorated the bosom of the moon god in one of the temples of the sacred Indian city of Somnaut. In the XI century, saving the statue from the Mohammedan conquerors, the three brahmins transported it to Benares. It was there that the gods of Vishnu appeared to the Brahmins in a dream, commanded them to guard the Moonstone day and night until the end of the century and foretold the misfortune of that daring who would dare to take possession of the stone and all his descendants to whom the stone would pass after him. The century passed after the century, the successors of the three Brahmins kept their eyes on the stone. At the beginning of the XVIII century. The Mongol emperor betrayed the plundering and ruining of the temples of the Brahma worshipers. Moonstone was abducted by one of the commanders. Not being able to return the treasure, the three guardian priests, disguised, watched him. The warrior who committed the sacrilege died. The moonstone passed, bringing with them a curse, from one illegal owner to another, the successors of the three priests continued to follow the stone. The diamond was in the possession of the Serangapatam sultan, who made him into the hilt of his dagger. During the assault by the English troops of Seringapatam in 1799, John Herncastle, without stopping before the murder, grabs a diamond.
Colonel Herncastle returned to England with such a reputation that the doors of his family were closed before him. The wicked Colonel did not value the opinion of society, did not try to justify himself, and led a solitary, vicious, mysterious life. Moonstone John Herncastle bequeathed his niece Rachel Verinder as a gift on the day of the eighteenth birthday. In the summer of 1848, a diamond brought from London to the estate of Verinder, Franklin Black, cousin Rachel, but even before his arrival near the house of Verinders, three Indians and a boy appear who pretend to be wandering magicians. In fact, they are interested in Moonstone. On the advice of the old butler Gabriel Betteredge, Franklin takes the diamond to the nearest bank in Frizinghall. Time before the birthday of Rachel passes without special events, Young people spend a lot of time together, in particular, painting the door of Rachel’s small drawing-room. In the feeling of Franklin to Rachel can not be doubted, her attitude to him is still unknown. Perhaps it’s more to her heart than her other cousin, Godfrey Eblewhite. On his birthday, Rachel Franklin brings a diamond from the bank. Rachel and the guests who have already arrived are overjoyed, only the mother of the girl, Milady Verinder, shows some concern. Before dinner Godfrey explains to Rachel in love, but receives a refusal. At dinner Godfrey is grim, Franklin is merry, excited and speaks out of place, without malicious intent, setting himself against those around him. One of the guests, Phryngoll Dr. Kandy, noticing Franklin’s nervousness and hearing that he recently suffers from insomnia, advises him to undergo treatment, but receives an angry rebuff. It seems that a diamond, which Franklin managed to attach to the dress of Rachel like a brooch, caused damage to those present. As soon as dinner was over, the sounds of the Indian drum were heard and magicians appeared at the porch. The guests wanted to see the tricks and poured out onto the terrace, and with them, and Rachel, so that the Indians could make sure that the diamond was in her possession. Mr. Murthwaet, a well-known traveler in India, also present among the guests, undoubtedly determined that these people are only disguised as jugglers, and in fact – the brahmins of a high caste. In a conversation between Franklin and Mr. Murthuet it turns out that the gift is a sophisticated attempt by Colonel Herncastle to harm Rachel, that the diamond owner is in danger. The end of the festive evening is no better than dinner, Godfrey and Franklin are trying to hurt each other, and in the end Dr. Kandy and Godfrey Ablewhite agree on something mysteriously. Then the doctor leaves home under the sudden pouring rain.
The next morning it turns out that the diamond was gone. Franklin, against the expectation of a good sleep, is actively starting to search, but all attempts to find a diamond lead to nothing, and the young man leaves for the police. The disappearance of the jewelery had a strange effect on Rachel: not only was she upset and nervous, her frankness towards Franklin was undisguised anger and contempt, she did not want to talk or see him. Inspector Seagrave appears in the house of Verinder. He searches the house and rudely interrogates the servants, then, without obtaining results, leaves to take part in the interrogation of the detainees on suspicion of the theft of a diamond of three Indians. From London comes the famous detective Cuff. It seems that he is interested in everything, except for the search for a stolen stone. In particular, he is not indifferent to roses. But here the detective notices a spot of smeared paint on the door of Rachel’s small drawing room, and this determines the direction of the search: on whose clothes the paint is found, he, therefore, took the diamond. In the course of the investigation, it turns out that Rosanna Spearman’s maid, who was serving milady from the correctional home, has been acting strange lately. On the eve of Rosanna met on the road to Frizinghall, and Rosanna’s merchandise attest that she had been burning all night, but she did not answer the knock at the door. In addition, Rosanna, irresponsibly in love with Franklin Black, dared to speak to him in an unusually familiar manner and seemed ready to tell him something. Cuff, interrogating the servants in turn, begins to follow Rosanna Spearman. Came with the butler Betteredge in the house of friends Rosanna and skillfully leading the conversation, Cuff guesses, that the girl hid something in the Shivering Sands – an amazing and terrible place not far from the Verinder estate. In the... quicksand, as in the quagmire, any thing disappears and a person may die. It is this place that becomes the resting-place of a poor suspected servant, who also had the opportunity to see her utter indifference to her and her fate as Franklin Black.
Milady Verinder, concerned about her daughter’s condition, takes her to her relatives in the freezing alleys, Franklin, having lost Rachel’s location, leaves first for London, then travels around the world, and Sergeant Cuff suspects that the diamond was stolen by Rosanna at the request of Rachel herself, and believes that soon the case of the Moonstone will come up again. The day after Franklin’s departure and the owners of the house, Betteredge encounters the Cripple of Lucy, Rosanna’s girlfriend, who brought the deceased’s letter to Franklin Black, but the girl does not agree to give the letter otherwise as an addressee in her own hands.
Milady Verinder and his daughter live in London. The doctors prescribed Rachel’s entertainment, and she tries to follow their recommendations. Godfrey Eblewhite in the opinion of the world is one of the possible abductors of the Moonstone. Rachel sharply protests against this accusation. Gentleness and devotion to Godfrey persuade the girl to accept his offer, but her mother is dying from a long-term heart disease. Father Godfrey becomes the guardian of Rachel, she lives with the Ablewain family in Brighton. After the visit of the lawyer Bruff, who has been dealing with family affairs for many years, and talking with him, Rachel dissolves her engagement, which Godfrey accepts without complaint, but his father suits the girl because of whom she leaves the guardian’s house and temporarily settles in the solicitor’s family.
Having received news of his father’s death, Franklin Black returned to London. He tries to see Rachel, but she stubbornly refuses to meet with him and receive his letters. Franklin leaves for Yorkshire, where the Verinders’ house is located, to try once again to uncover the mystery of the disappearance of the Moonstone. Here, Franklin’s letter is transmitted to Rosanna Spearman. A brief note contains instructions, following which Franklin pulls out of the Shifting Sands a secret nightclub hidden in the cache, stained with paint. To the deepest amazement, he discovers his mark on the shirt! And Rosanna’s death message with the shirt in her cache explains the feelings that made the girl buy cloth, sew a shirt and replace it with the one that was smeared with paint. With difficulty accepting the incredible news – that it was he who took the diamond, “Franklin decides to bring the investigation to an end.” He manages to persuade Rachel to talk about the events of that night. It turns out that she saw with her own eyes how he took the diamond and left the small living room. Young people part in sadness – an undisclosed mystery stands between them. Franklin decides to try to repeat the circumstances that preceded the loss of the stone, in the hope of tracing where he could have gone. It is impossible to gather all those present at the birthday of Rachel, but Franklin asks about the events of the memorable day of all who can be found. Having arrived on a visit to Dr. Kandy, Franklin is struck by the change that has taken place in him. It turns out that the cold, caught by the doctor on the way home from guests about a year ago, went into fever, as a result of which the memory now and then brings Mr. Kandy, which he diligently and vainly tries to hide. Assistant doctor, Ezra Jennings, a sick and unhappy man, taking part in the fate of Franklin, shows him the diary entries made when Jennings was courting the doctor at the very beginning of the illness. Comparing these data with eyewitness accounts, Franklin realizes that he was given a small dose of opium in his drink (Dr. Kandy did not forgive him ridicule and wanted to laugh at him in his turn), and this, overlapping his concern about the fate of the stone and the nervousness associated with by the fact that he had recently quit smoking, plunged him into a state like a lunatic. Under the guidance of Jennings, Franklin prepares himself for a repetition of the experience. He again quits smoking, he again begins insomnia. Rachel secretly returns to the house, she again believes in Franklin’s innocence and hopes that the experience will be successful. On the appointed day, under the influence of a dose of opium, Franklin, as well as the last time, takes a “diamond” (now it is replaced by a glass of about the same kind) and carries it to his room. There glass falls out of his hands. Franklin’s innocence is proven, but the diamond has not yet been found. Traces of it are soon found: an unknown bearded man buys a certain jewel from the pawnbroker Luke, whose name and earlier rumor associated with the history of the Moonstone. The man stops at the Fortune’s Tavern, but Franklin Black, along with Sergeant Cuff, finds him already dead. After removing the wig and false beard from the deceased, Cuff and Franklin recognize Godfrey Eblewhite in it. It turns out that Godfrey was the guardian of one young man and squandered his money. In a desperate situation, Godfrey could not resist, when Franklin, in unconsciousness, gave him a stone and asked him to hide it better.
Misunderstandings between Franklin and Rachel are forgotten, they marry and live happily. Old Gabriel Betteredge watches them with pleasure. A letter comes from Mr. Murthuet, in which he describes a religious ceremony in honor of the god of the Moon, which took place not far from the Indian city of Somnaut. The traveler completes the letter describing the statue: the moon god sits on the throne, his four arms are stretched to the four sides of the world, and a yellow diamond shines in his forehead. The lunar stone after the passage of centuries again found itself within the walls of the sacred city, where its history began, but it is not known what other adventures may fall on its share.