They say there are only twenty or so. Plots can be boiled down to certain themes: coming of age, finding love, restoring justice, etc. The devil, as they say, is in the details.
Mysteries tend to be about restoring--or at least seeking--justice, so the reasons we choose and read certain mysteries will often relate to subthemes. Many like mysteries where they learn about a job, a craft, a way of life. I like some of that, too, but I also want some creativity in the way the protag looks for justice.
I just finished a book that was okay. There was a subtheme about a certain job/lifestyle, and there was lots of interesting info about how people operate in that setting. The mystery, however, was so trite as to be irritating, and the human interest part of the story so predictable that I was thinking, "Okay, in this chapter she's going to fight with her boyfriend again, so I'll just skip to the next one and get back to the story."
It's a problem with genre fiction, and the reason I seldom read romances. The reader knows how the story goes as well as the writer, so there isn't any tension.
On the other hand, there are mysteries (and romances, too) that break the mold and make the old search-for-justice theme new and interesting. Finished with (mostly skimming) the book mentioned above, I began a new book where I was pulled into the story right away. I don't know what's going to happen, but I want to know, because I care about the characters and the author has skillfully led me into their lives. I'm liking it a lot, though I'm only about ten chapters in.
You want to know what it is, don't you? It's called AN UNCERTAIN REFUGE by Carolyn Rose.
Now, I might come back next week and tell you I was disappointed in the ending--don't know, haven't got there yet. But I really won't mind, because I'm getting my money's worth along the way. Plots are indeed limited to certain themes, but a good author takes that theme and makes a worthwhile story of it.
And that's what I'm talking about!