The aristocratic Charles Bromley arrives at 221B Baker Street to beg the great detective for his help. Bromley believes that his wife is in danger, as she has refused an offer to sell the Moonstone, a fabulous diamond that has been in her family for generations but which is said to be cursed. When a jeweller is found murdered, it seems as if the Moonstone deserves its reputation. Then the diamond is stolen, and Holmes must try to unravel a mystery centuries in the making.