The Moonstone, a priceless Indian diamond which had been brought to England as spoils of war, is given to Rachel Verrinder on her eighteenth birthday. That very night, the stone is stolen. Suspicion then falls on a hunchbacked housemaid, on Rachel's cousin Franklin Blake, on a troupe of mysterious Indian jugglers, and on Rachel herself.

The phlegmatic Sergeant Cuff is called in, and with the help of Betteredge, the Robinson Crusoe-reading loquacious steward, the mystery of the missing stone is ingeniously solved.