Let's talk about fiction mystery. It defines everything from detective stories to supernatural stories. Mystery means "something not understood; a problem or puzzle"; mysterious meaning "impossible or difficult to comprehend", and of course fiction meaning "something that is created or imaginary; a literary work that is produced by the imagination and not based on fact."
A well written mystery, whether it be a story, poetry, or anything else, captures the reader's interest from beginning to end, and is not predictable. The perfect mystery leaves a reader in suspense the entire time and is like figuring out a puzzle. If the piece is well written, the finale will come as a shock to a reader.
There's nothing more monotonous that predictability in a mystery story.
Strong character definition and representation of the place the mystery began are also imperative. If a reader cannot relate to the characters, more than likely interest will be lost. The same applies to the place where the event occurs. For example, if the mystery begins in a ghost town, make sure the town is well defined in a mysterious way that will lure a reader in instantly.