Moonstone - a huge yellow diamond - from time immemorial adorned the brow of the moon god in one of the temples of the holy Indian city of Somnaut. In the 11th century, saving the statue from the Mohammedan conquerors, three Brahmins transported it to Benares. It was there that the god Vishnu appeared in a dream to the brahmins, ordered them to guard the Moonstone day and night until the end of the century, and foretold the misfortune of the daring one who dares to take possession of the stone, and all his descendants, to whom the stone will pass after him. The century passed after century, the successors of the three Brahmins did not take their eyes off the stone. At the beginning of the XVIII century. the Mongol emperor betrayed the robbery and ruin of the temples worshipers of Brahma. Moonstone was stolen by one of the military leaders. Not being able to return the treasure, the three guardian priests, having changed clothes, watched him. The warrior who committed the sacrilege died. The moonstone passed, bringing with it a curse from one illegal owner to another, the successors of the three priests continued to monitor the stone. The diamond was in the possession of the Seringapatam Sultan, who embedded it in the hilt of his dagger. During the assault by the British troops of Seringapatama in 1799, John Herncastle, not stopping before the assassination, captures a diamond.

Colonel Gernkastl returned to England with such a reputation that the doors of his family were closed in front of him. The wicked colonel did not cherish the opinion of society, did not try to make excuses and led a solitary, vicious, mysterious life. Moonstone John Gernkastl bequeathed to his niece Rachel Verinder as a gift for the day of the eighteenth anniversary. In the summer of 1848, Franklin Black, Rachel's cousin, brought diamond from London to the Verinder estate, but before his arrival, three Indians and a boy appeared near the Verinder house, posing as wandering magicians. In fact, they are interested in Moonstone. On the advice of old butler Gabriel Betteredge, Franklin takes the diamond to the nearest bank in Friesingall. The time before Rachel’s birthday passes without special events. Young people spend a lot of time together, in particular, painting the door of Rachel’s small living room with patterns. Franklin’s feeling for Rachel has no doubt, her attitude towards him remains unknown. Perhaps she cherishes her other cousin, Godfrey Eblewight. On his birthday, Rachel Franklin brings a diamond from the bank. Rachel and the already arrived guests are beside herself with delight, only the girl’s mother, Milady Verinder, shows some concern. Before dinner, Godfrey Rachel declares her love, but is refused. At lunch, Godfrey is gloomy, Franklin is cheerful, excited and speaks out of place, without malicious intent, setting others against himself. One of the guests, Frisingoll Dr. Kandy, noticing Franklin's nervousness and hearing that he has recently suffered from insomnia, advises him to undergo treatment, but receives an angry rebuke. It seems that the diamond that Franklin managed to attach to Rachel’s dress like a brooch, spoiled those present. As soon as dinner was over, the sounds of an Indian drum were heard and magicians appeared at the porch. The guests wished to see the tricks and poured onto the terrace, and with them Rachel, so that the Indians could make sure that she had the diamond. Mr. Mertuet, a well-known traveler in India, also present among the guests, no doubt determined that these people were just dressed as magicians, and in fact - Brahmins of high caste. In a conversation between Franklin and Mr. Mertuet, it turns out that the present is a sophisticated attempt by Colonel Gernkastl to harm Rachel, that the owner of the diamond is in danger. The end of the festive evening is no better than lunch, Godfrey and Franklin try to hurt each other, and in the end, Dr. Kandy and Godfrey Eblewight mysteriously agree on something. Then the doctor leaves home in the sudden pouring rain.

The next morning it turns out that the diamond is gone. Franklin, who had slept well against anticipation, is actively embarking on a search, but all attempts to find a diamond have no effect, and the young man leaves for the police. The loss of the jewel had a strange effect on Rachel: not only was she upset and nervous, she showed open anger and contempt in relation to Franklin, she did not want to talk to him or to see each other. Inspector Seagrave appears in the Verinder House. He searches the house and interrogates the servants rather rudely, then, having not achieved results, leaves to take part in the interrogation of the three Indians on suspicion of stealing a diamond. The famous detective Cuff arrives from London. He seems to be interested in everything except searching for the stolen stone. In particular, he is not indifferent to roses. But then the detective notices a speck of smeared paint on the door of Rachel’s small living room, and this determines the direction of the search: on whose clothes the paint is found, he, therefore, took a diamond. During the investigation, it turns out that the maid Rozanna Spearman, who entered the service of the milady from the correctional house, has recently behaved strangely. On the eve of Rosanna, she was met on the road to Freezingall, and Rosanna’s goods showed that she had been burning all night, but she did not answer the knock on the door. In addition, Rosanna, unrequitedly in love with Franklin Black, dared to speak with him in an unusually familiar manner and seemed ready to tell him something. Cuff, interrogating the servants in turn, begins to follow Rosanna Spearman. Caught with Butler Betterge at the house of Rosanna's friends and skillfully having a conversation, Cuff realizes that the girl hid something in Quicksand - an amazing and terrible place near the Verinder estate. In Quicksand, like in a quagmire, any thing disappears and a person may well die. It is this place that becomes the resting place of the poor suspect maid, who also had the opportunity to be convinced of the complete indifference to her and her fate, Franklin Black.

Milady Verinder, worried about her daughter’s condition, takes her to relatives in Friesingall, Franklin, having lost Rachel’s location, first leaves for London, then travels around the world, and detective Kuff suspects that the diamond was stolen by Rosanna at the request of Rachel herself, and believes that soon the Moonstone case will come up again. The day after Franklin and the owners of the house left, Betterage met Lromo the Lucy, Rosanna’s girlfriend, who brought the letter to the deceased for Franklin Black, but the girl does not agree to hand the letter to the addressee.

Milady Verinder with her daughter live in London. Doctors prescribed Rachel entertainment, and she is trying to follow their recommendations. Godfrey Eblwight in the opinion of the world - one of the possible captors of the Moonstone. Rachel sharply protests against this charge. The meekness and devotion of Godfrey incline the girl to accept his offer, but then her mother dies from a long-standing heart disease. Father Godfrey becomes the guardian of Rachel, she lives with the Ebwight family in Brighton. After the visit of solicitor Breff, who has been involved in family affairs for many years, and a conversation with him, Rachel terminates his engagement that Godfrey accepts meekly, but his father makes a scandal for the girl because of which she leaves the guardian’s house and temporarily settles in the family of the solicitor.

Upon receiving news of the death of his father, Franklin Black returns to London. He tries to see Rachel, but she stubbornly refuses to meet with him and accept his letters. Franklin leaves for Yorkshire, where the Verinder House is located, to once again try to uncover the mystery of the disappearance of the Moonstone. Here Franklin is handed the letter to Rosanna Spearman. The brief note contains instructions, following which Franklin pulls out of the Quicksand hidden in his cache the nightgown stained with paint. To his utter amazement, he finds his mark on his shirt! And Rosanna’s dying letter, which was with the shirt in the cache, explains the feelings that made the girl buy fabric, sew a shirt and replace it with one that was smeared with paint. Having hardly received the incredible news - that it was he who took the diamond - Franklin decides to complete the investigation. 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 sorrow - an undisclosed secret stands between them. Franklin decides to try to repeat the circumstances that preceded the disappearance of the stone, in the hope of tracing where he could go. It is impossible to collect all those present at Rachel's birthday, but Franklin asks about the events of the memorable day to everyone he can find. Arriving on a visit to Dr. Kandy, Franklin was amazed at the change that had occurred in him. It turns out that the cold, picked up by the doctor on the way from the guests home about a year ago, went into a fever, as a result of which the memory now and then brings Mr. Kandy, which he is carefully and vainly trying to hide. The doctor’s assistant, Ezra Jennings, a sick and unhappy man, taking part in Franklin’s fate, shows him the diary entries made when Jennings took care of the doctor at the very beginning of the illness. Comparing these data with eyewitness accounts, Franklin understands that a small dose of opium was mixed in with him (Dr. Kandy did not forgive him the ridicule and, in turn, wanted to laugh at him), and this, overlapping with his concern about the fate of the stone and the nervousness associated with by the fact that he recently quit smoking, plunged him into a state similar to sleepwalking. Under the direction of Jennings, Franklin prepares himself to repeat the experience. He quits smoking again, and insomnia begins again. Rachel secretly returns to the house, she again believes in Franklin's innocence and hopes that the experiment will be successful. On the appointed day, under the influence of a dose of opium, Franklin, like the last time, takes a “diamond” (now it is replaced by glass of about the same type) and takes it to his room. There the glass falls out of his hands. Franklin's innocence is proven, but the diamond has not yet been found. Traces of it will soon be revealed: an unknown bearded man buys a certain jewel from a loan shark, Lucer, whose name was previously rumored to be connected with the history of the Moonstone. The man stops at the Wheel of Fortune tavern, but Franklin Black, who arrived there, along with detective Cuff, find him already dead. Having removed the wig and the fake beard from the deceased, Cuff and Franklin recognize Godfrey Eblewight in it. It turns out that Godfrey was the guardian of a young man and wasted his money. In a desperate situation, Godfrey could not resist when Franklin unconsciously gave him a stone and asked him to hide better. Feeling complete impunity, Godfrey mortgaged the stone, then, thanks to a small inheritance, bought it, but was immediately discovered by the Indians and killed.

The misunderstandings between Franklin and Rachel are forgotten, they get married and live happily ever after. Old Gabriel Betterge gladly watches them. A letter comes from Mr. Mertuet, in which he describes a religious ceremony in honor of the moon god, taking place near the Indian city of Somnaut. The traveler concludes the letter with a description of the statue: the moon god sits on the throne, his four arms are extended to the four cardinal points, and a yellow diamond shines in his forehead. Over the course of centuries, the moonstone was again within the walls of the holy city, where its history began, but it is not known what other adventures may fall on its lot.