The “Moonstone” of the title is a large but flawed diamond, looted from India at the time of the Mutiny by an unscrupulous British officer. Many years later, estranged from his family due to his licentious lifestyle, the officer bequeaths the diamond to his sister’s daughter, Rachel Verrinder, to be given to her on her 18th birthday. Due to the ill-omens surrounding the gem, this may have been an act of revenge rather than reconciliation. The diamond, it appears, was taken from a statue of the Moon God worshipped by a Hindu cult, and it has long been sought by a group of Brahmins determined to return it to their temple.
On the night of the birthday party the gem mysteriously disappears from Rachel’s room. While the first suspicions naturally fall on these Indians, they are eventually exculpated. Rachel becomes hysterical and angry when questioned about the theft and refuses to assist the police. Active efforts to assist them are taken up by Rachel’s cousin (and sweetheart) Franklin Blake. These efforts simply drive Rachel into further fury, and she becomes completely estranged from him. Suspicion thus falls on her as having some secret reason for wishing to raise money on the diamond. The novel proceeds to slowly uncover the mysteries involved.