In her essay “Who’s Doing It? Fifteen Years of Work on Victorian Detective Fiction,” Anne Humpherys outlines the plot structure of the classic detective story:

In the classic detective story there is a crime, a series of possible clues (not all, of course, are relevant), and a figure-either a police or private investigator-who steps in, sorts out the clues by working backwards from the present, and then provides the “true” solution, namely a theory that arranges everything in the only “correct” way. In other words, in most cases, without a detective there would be no detective story.