"The Detective Detector" is a Sherlock Holmes parody by the American author William Sydney Porter who wrote under the pseudonym of O. Henry. The short story was first collected in the 1917 anthology Waifs and Strays.
In the story, a master criminal named Avery Knight takes on the challenge of predicting where a detective on a murder case will go to look for the murderer. He commits a random murder then tries to anticipate the moves of the great detective Shamrock Jolnes by reverse-engineering the detective's famous methods. When that approach fails, Knight decides to follow his own theory instead.
The unnamed narrator is taking a walk in Central Park with the great New York criminal Avery Knight. Knight boasts that he can commit a murder and, within 48 hours, take the narrator to the detective assigned to the case. Finding the narrator skeptical, Knight decides to demonstrate. He draws his revolver and shoots a man walking ahead of him. He then takes the man's money and jewelry. As they walk away, Knight and the narrator see a policeman running towards the spot to check on the shot. Knight stops the policeman and tells him that he just killed and robbed a man. The policeman angrily tells Knight to get out of the park.
The following afternoon, Knight visits the narrator to report his progress. He has learned that his card case containing his calling cards with his address was found at the scene. He also heard three witnesses give his description at the inquest. Shamrock Jolnes, the famous detective, has been assigned to the case. Knight says he waited at his address for a few hours but Jolnes did not come. The narrator believes Jolnes is off chasing his theories and will not be found. Undeterred, Knight promises to take him to see the famous detective the following day.
Avery Knight returns the next day and admits to the narrator that he has failed so far to find Jolnes. Knight is familiar with Jolnes' methods, so he has tried to anticipate the detective's moves. Since the weapon was a .45-caliber pistol, he has looked for Jolnes in 45th Street. Since he shot the man in the back, which suggests a possible hazing, he has been to the Columbia University. The narrator points out that it is nearly impossible for a criminal and the detective in charge of the case to meet each other in New York City. Knight considers the matter for a long time then finally declares the problem solved.
Knight takes the narrator on a cab ride. The cab goes up Fifth Avenue and stops in front of a beautiful residence. They find Shamrock Jolnes in a false beard watching the house. The narrator, amazed at the brilliant feat, asks by what process of induction Knight arrived at the solution.
Knight tells the narrator that, while the detectives use the inductive theory, he himself uses the saltatorial theory instead. Rather than try to solve a mystery from small clues, he jumps straight to a conclusion. Since homicides go unsolved in New York despite the efforts of the best detectives, he concluded that the detectives must go about their work in the wrong way – or the exact opposite of the right way.
Knight describes himself as a tall man with a black beard. He says he hates publicity, is a poor man with an ambition to become rich, and he never gives to charity. Therefore, Knight assumed that Jolnes would look for the exact opposite – a short man with a white beard who is a publicity-loving wealthy philanthropist – and brought the narrator to the residence of Andrew Carnegie.