Plot, by definition, is the main events in a story or play. A key thing to note here is that plot shapes the story like the framework for a house. Details should be filled in but without logical framework the structure will collapse.
Something that I see during the beta reading process is the trap of directionless plot. To continue the construction metaphor, the author gets so fixated on putting one piece of framing in a particular space that he fails to notice how it might affect the entire structure.
Every piece of plot should build off each other. Do the characters really need to explore that mysterious tomb? Or do you just want to have a scene where it might be cool to fight the living dead? How does exploring that tomb add to the other plot points?
Now, this is not to say that there cannot be subplots that veer away from the main plot. In longer stories, subplots and character development are what make a story memorable. But the main framework should always be in focus. A reader should not have to read 75% of a book in order to finally figure out what it is that the characters are trying to accomplish.