Fair Play
Readers find it enjoyable to play detective. It’s up to the writer to decide whether or not this game will be fair? Will the story contain all the necessary clues for the reader to solve the mystery?
Fair

A mystery is considered fair if all the clues are presented to the reader. Of course, there are many ways to present clues. One way to play fair is to lay out all the clues on the table, making them very obvious. Mystery readers typically don’t enjoy solving puzzles when they’re too easy.
Unfair

Some writers hide crucial information from the reader, allowing only the sleuth to solve the mystery. For example, imagine a detective solving a case because they knew about the suspect’s twin brother—when the story never mentioned the sibling. Since there was no way for the reader to deduce this twist, it can leave them frustrated.
While many detectives have specialized skills and knowledge that the reader lacks, readers still want to feel like they are along for the ride and privy to the sleuth’s reasoning. Sometimes, as in the case of Sherlock Holmes, the writer seems less concerned with playing fair and more focused on showcasing the detective’s brilliance.
The Sweet Spot

Both extremes of the fair play scale can result in an unpleasant reader experience. Mysteries with obvious clues don’t challenge readers enough. Mysteries with impossible clues frustrate readers.
Readers want to feel smart. The ideal mystery ensures that all the clues are present but cleverly hidden. If a reader were to go back and reread the story, they should be able to spot all the clues they missed the first time.
Fair, or not, every mystery story follows the same four steps.