These definitions give away parts of the plot to Sir Arthur Conan Doyle's Sign of 4.
A Detective Story
A work of fiction in which a detective, either a policeman or private citizen solves a puzzling crime. Detective stories can be novels or short stories.
An unfortunate person who suffers from a crime or disaster (Obeť).In The Sign of Four, Captain Morstan and Bartholomew were victims.
Someone who sees a crime or has information about it (Svedok).
Maj. Sholto was a witness to Capt. Morstan's death (and possibly the killer). Thaddeus and Bartholomew were witnesses to Maj. Sholto's death. And, Mrs. Bernstone was a witness to Bartholomew's death.
Anyone who might have committed a crime (Podozrivý človek). A good detective story will have several false suspects who are innocent, and sometimes a “least likely suspect” with no apparent motives.
In The Sign of Four, Thaddeus is a suspect, but he's innocent.
A reason for committing a crime.
An Inside Job
A crime committed by or with the help of a family member, employee, and/or someone trusted by the victim.
A red herring is an idiom for a clue which is intentionally or unintentionally misleading or distracting from the actual crime. [The idiom probably originates from an article published 1807 by journalist William Cobbett in the Political Register. In a critique of the English press, which had mistakenly reported Napoleon's defeat, Cobbett recounted that he had once used a red herring to deflect hounds in pursuit of a rabbit.]
A Private Investigator, Private Eye
Someone independent from the state who assists either the police or a victim’s family in solving a crime. They’re celebrated for their logic and intellect. Sherlock Holmes is a private eye.
A Criminal Mastermind
The villain present in some detective stories, a criminal genius who commits many crimes, yet is not suspected by the police. He/she is usually wealthy and respected. The Criminal Mastermind in the stories of Sherlock Holmes is Prof. Moriarty, but he isn't mentioned in this story. In this story, the criminals, Jonathan Small and Tonga, are not masterminds, just average.
A Bungling Local Constabulary
This is an incompetent policeman who needs the help of a private eye. To bungle is to make a mistake, or to trip and fall down, for example during a sports match. The bungling police detective in this story is Lestrade.
An inquiry is a conversation in which the private eye asks a series of questions to witnesses and suspects in order to determine who is guilty.
A Reconstruction of the Crime
In an effort to find who is guilty of a crime, the detective often recreates the scene, asking witnesses and suspects to reenact what they did exactly as they remember it. It helps to find clues, and to see who is lying.
A Plot Twist
A surprising revelation (Zápletka). Some "twists" are foreshadowed and can be predicted, whereas others are a complete shock. When Bartholomew is found dead, it's a plot twist, as is the discovery of Tonga.
A "Locked Room" Mystery
This is a detective story in which a crime—almost always murder—is committed under apparently impossible circumstances. It typically involves a crime scene that no intruder could have entered or left, for example, a locked room. The reader is normally presented with the puzzle and all of the clues, and is encouraged to solve the mystery before the solution is revealed in a dramatic climax. This format is commonly referred to as “whodunit”,meaning “who did it?” Bartholomew's murder is a locked room mystery, but quickly solved by Sherlock Holmes.
An Inverted Detective Story
An inverted detective story is the opposite of a “whodunit”. The identity of the criminal is described at the beginning. The remainder of the story then describes the subsequent investigation. The puzzle presented to the reader is discovering the clues and evidence that the perpetrator left behind, in order to understand why they did it. A famous example is In Cold Bloodby Truman Capote.