Inspector Gabriel Hanaud, a stout professional policeman from the French Sûreté, is called upon to investigate the murder of a French aunt by her young English niece in Dijon. Unlike Sherlock Holmes, Hanaud relies on psychological insights rather than physical evidence in his investigations. The retired London banker, Mr. Julius Ricardo, serves as his "Watson", though he appears only briefly in this novel. Inspector Hanaud is the opposite of Conan Doyle's Sherlock Holmes and is said to have been the model for Agatha Christie's Hercule Poirot.