In detective fiction, a reader follows the protagonist through a complex and challenging murder-mystery storyline. The plot involves uncovering clues; questioning witnesses; and solving puzzles to learn the identity of the antagonist, the original perpetrator of the crime; and trying to catch him before he can strike again. Detective fiction, like other fiction genres, involves some combination of a few popular elements such as key character archetypes and investigative or puzzling plot devices.
The protagonist fills the archetype of the detective, an investigator who may be an amateur, such as Agatha Christie's Miss Marple; a self-employed private investigator such as Sherlock Holmes; or a law-enforcement officer. The detective usually possesses superior powers of deduction and reasoning in comparison to those around her. In "The Ten Commandments of Detective Fiction," an article on the website MysteryList, S. S. Van Dine, a popular detective-fiction author from the 1920s, wrote that the reader must uncover clues and unravel puzzles at the same rate as the detective, and that the writer cannot play special tricks on the reader to extend the mystery or suspense without also playing them on the detective as well.
Although no longer as popular as it once was, the Watson remains a core element of detective fiction. Based upon the best friend and chronicler of Arthur Conan Doyle's Sherlock Holmes, the Watson usually emerges as a less-intelligent, more bumbling friend or co-worker of the story's detective figure, one who challenges or questions the Detective on issues related to the investigation and occasionally presents red-herrings for the reader and Watson to chase while the detective pursues the real culprit. A teacher's guide for "The Hound of the Baskervilles" (see PBS link in References section) says that the Watsons of traditional detective fiction have intelligence "slightly, but very slightly, below that of the average reader."
A good detective story "centers around the question of 'who done it,'" according to a Yale-New Haven Teacher's Institute curriculum guide for detective fiction. This question "keeps the readers' attention, building excitement from the elimination of several suspects to the surprise ending." The antagonist, or criminal opponent of the detective and the Watson, provides the answer to the question of "who done it." The individual or a conspiracy's actions throughout the story attempt to frustrate or dissuade the detective's investigation. In order for the story to remain interesting and exciting, the antagonist's attempts to cover her tracks must appear to resemble a perfect crime.
The plot line of any good detective story follows the course of the detective's investigation. In some plot lines, the solving of the murder mystery exposes an individual who was not previously a suspect. In other stories, solving the mystery prevents the antagonist from committing further crimes. The PBS teacher's guide for "The Hound of the Baskervilles" says that as the story concludes, "the detective grasps the solution to the crime long before anyone else, and explains it all to the 'Watson' at the end."