A legendary diamond, the Moonstone, is stolen from a young woman's bedroom on her birthday, igniting a complex investigation into its disappearance. Rachel Verinder's inheritance of the sacred Indian diamond, the Moonstone, marks her for misfortune, as the gem is stolen on the night of her birthday party. The narrative unfolds through a series of journal entries and testimonies from various characters, including Rachel, her cousin Franklin Blake, the detective Sergeant Cuff, and the servants, each offering a piece of the puzzle. The primary conflict lies in uncovering the thief among a household rife with secrets, conflicting loyalties, and hidden motives, all while navigating the superstitions surrounding the diamond's curse.

The Moonstone is widely regarded as a seminal work in the mystery genre, often cited as one of the first true detective novels. Critics and readers alike have praised its intricate plot, its innovative use of multiple narrators, and its detailed portrayal of Victorian society. The novel's enduring appeal lies in its complex characters and its masterful construction of suspense, making it a foundational text for subsequent crime fiction. Its status as a classic is cemented by its continued study and appreciation for its literary merit and its contribution to the development of the mystery novel.