Major John Sholto dies of a heart attack after a man with a wooden leg breaks into his house. Before his death Major Sholto revealed to his two sons that a fortune in jewels is hidden in the attic of the house. He also revealed how, six years ago, he murdered a Captain Morstan in order to be the sole owner of the jewels. Now Captain Morstan's daughter, Mary, is seeking Sherlock Holmes's help: she has been asked to meet an anonymous stranger, a stranger who just sent her the second largest diamond in the world.