"The Moonstone" is a detective novel that revolves around a large, valuable yellow diamond that was stolen from an Indian temple and is now in England. The diamond is bequeathed to a young woman on her eighteenth birthday, but is stolen that same night. The novel follows the investigation of the theft, which is complicated by a series of confusing events and false leads. The resolution involves the unraveling of a tangled web of deception, crime, and colonial guilt.

In this gripping installment of a renowned detective series, the protagonist, a shrewd and seasoned detective, finds himself embroiled in a complex case that intertwines a mysterious death, which at first glance appears to be a suicide, with the intricacies of a local amateur dramatic society's production of a medieval mystery play. As the detective delves deeper into the investigation, he is confronted with a series of cryptic messages and puzzles, challenging his analytical skills and drawing him into a dangerous game of cat and mouse with a cunning adversary. Set against the backdrop of a small English town, the narrative weaves together themes of passion, betrayal, and the quest for justice, all while exploring the moral ambiguities that lie at the heart of human nature.