mystery vs mystery story

mystery and mystery story both are nouns.

Word	Noun	Adjective	Verb	Adverb
mystery	Yes	No	No	No
mystery story	Yes	No	No	No

As nouns, mystery and mystery story are synonyms defined as:

mystery and mystery story: a story about a crime (usually murder) presented as a novel or play or movie

Other synonyms of mystery include whodunit.

mystery (noun)	mystery story (noun)
something that baffles understanding and cannot be explained	a story about a crime (usually murder) presented as a novel or play or movie
a story about a crime (usually murder) presented as a novel or play or movie