Franklin Blake is in love with Rachel Verinder. The gem he gives her, the moonstone, disappears, and the police cannot solve the crime. Eventually, Rachel spurns Franklin, and he goes abroad in an attempt to forget her and recover. When Franklin's father dies and Franklin returns to England, he resolves to win her back by solving the mystery of the moonstone.
Mademoiselle Stangerson retires to bed in the Yellow Room. Suddenly revolver shots echo through the house and she screams for help. Her father and a servant run to the locked room where they find the wounded girl - alone. The only other exit, a window - barred. How had the assailant escaped?