The most important part of a narrative story is the plot. It is the central focus of your writing and the reason you write in the first place. You are trying to fit a plot to your story to fulfill it and complete it. If you have a weak or incomplete plot, there is only so much to be done for the writing.
Everything in your story exists to serve a purpose; to tell the story of the plot. This means you may come to aspects of your writing with a degree of callous. You can not let your emotions get in the way. Whatever the plot demands, you have to provide. If the plot says something good happens to a character, let that something good happen. If the plot demands a character die, you must kill it. Your job as the writer is to tell the story presented by the plot line you have chosen, and you can not do that if you compromise the story.
It’s generally not a good idea to go into a story with no clue of where you will wind up. This is because you will be striving to write a story about nothing, which is not possible. It’s been attempted before and a lack of storyline leads things to fall apart at the seams. I personally like to start with a few sentences describing the story; a treatment. You then refine the treatment until you have chapters and a breakdown of pivotal points, and then outline the story.
Does this mean your plot line has to be bold and strong, firm and inflexible? Absolutely not. Even if you are writing a series of unrelated events, or “episodes” of a story with no interconnecting plot, you are making a narrative to follow in these mini stories. And if you’re thinking experimental writing, well, there is a difference between narrative storytelling and essay writing.
In short: Your plot line can be malleable or firm, bold or simple. Whatever you choose, a narrative story is nothing without it.