Incidents, events, no matter how fantastic or dramatic, do not constitute a plot. Mistake - You give the reader scenes, even brief ones, that provide no material contribution to the plot. The reader does not need to know about things that are irrelevant to the plot. For example, don't tell us how your lead character woke up one morning, brushed his teeth, took a shower, and did the laundry, unless that section is laced with important insights into the problem that's to come. Mistake - The novel contains a terrific coincidence.
Coincidences are the substance of everyday life, but they can ruin a novel.
If there must be a serious coincidence in your novel, let it appear as a coincidence until it proves itself to be part of a greater master plan. Coincidences are okay so long as they reveal themselves later to be part of the web woven by one or more of your characters. Susan knew them and believes they were murdered. But she can't prove it. She searches for the truth.