Write a list of all of the main characters of the story that will take part in the plot. You can include secondary and minor characters if you wish, but only the main characters are essential when you're working on the plot (since these are the characters that will be moving the plot along or who will be otherwise affected by it.)
Make notes of the major plot points of your story and which of your main characters will be involved in them. This doesn't have to be everything that happens in the story; you're only looking for the major events and the climax at this point. Further plot details will be filled in later.
Add secondary plot points to your list, indicating which major plot points they occur between or contribute to. Your list of secondary plot points will most likely be longer than your list of major plot points, since small plot points occur more often and are either the cause of or are caused by the major points. Include the introduction of new characters that are important to the plot in this list if they aren't introduced during major plot points.
Put plot points that are important to the details of the story on the list as well, even if they aren't very important to the story as a whole. These points should include anything that helps to set the scene as well as the basics of the opening and ending of the story.
Organize your list in the order that the plot points will appear in the story. This will give you a rough chronological view of what the story will look like plot-wise, as well as the characters that will be involved in each point of the plot. From here you will have an easy reference as to what happens next in the story and who will be involved in it, making the actual writing of the story much easier.