What is the best way to write a summary of the plot of a story?
Stories have an overall plot with sub-plots and side story lines. Try writing down what seems significant to you from any part of the story. Then, try to narrow it down to the main plot. Your summary of the main plot can mention sub-plots, but the summary needs to describe the main plot,
One of the best ways to figure out the plot of the story is to go in sequences--in other words, use a cause and effect sheet. If you want to improve upon this, you can even go so far as to write short summaries of significant sections of the story which you are working on. However, this method does not work for everyone. Try it for yourself and see how you like it, and then try to come up with your own ways. Everyone has their own comfort zone with this, so there is a bit of wiggle room--keep trying things. You'll figure out what you like the best eventually.