A good mystery story needs a mystery, of course. This could be a missing item, a strange event, or a person's unexplained disappearance. Also, interesting characters are key, like a detective or a suspect. And clues, they are essential to keep the reader guessing and trying to solve the mystery.
First, you need a setting that adds to the mystery. An old, spooky mansion or a fog - covered forest would be great. Then, there should be a conflict, something that makes the situation tense. Suspense is also very important. You build it by delaying the revelation of important information. And don't forget a plot twist at the end to surprise the reader.
Well, there are several elements. A mystery story should have a hook at the beginning to draw the reader in. Then, there must be a mystery that is not too easy to solve. The characters should have motives and secrets. Red herrings are also a good addition. These are false clues that mislead the reader. And finally, a satisfying resolution that ties up all the loose ends.