Although P.G. Wodehouse writes principally about the upper classes and the aristocracy, many of his characters are impecunious. Leave it to Psmith centers around a plot to steal a valuable necklace from Lady Constance Keeble, a wealthy woman whose brother is the Earl of Emsworth, owner of Blandings Castle. Although Lord Emsworth and his family have plenty of money, this money is under the control of a few people, including Lady Constance, who uses it to manipulate her family circle, particularly her husband, the mild-mannered Mr. Keeble.

Lord Emsworth’s younger son, Freddie Threepwood, needs money to pay gambling debts and become a bookmaker. Mr. Keeble needs money to buy a house for his stepdaughter and her husband. Psmith simply needs money, as he is unemployed. The three of them make a plan to steal Lady Constance’s necklace, replace it with a duplicate, and sell the original, solving the financial problems of all three men at a stroke. However, there are also professional thieves targeting the necklace, and by the time the story is thoroughly underway, it seems as though most of the residents of Blandings Castle are thieves or imposters, or both. Lack of money drives the characters to all manner of subterfuge, while possession of it makes Lady Constance even more tyrannical than she would otherwise be. Only Lord Emsworth, a simple man who wants only peace, quiet and comfort, remains unaffected.