According to Kelly O’Dell,

The marketing plan is a communication tool used to give direction to the company. It is not a checklist actions or a demonstration of the marketers analytical prowess. You might think of the marketing plan as a menu describing the food that will be served at a fancy party. This description tells you what will be served and in what order. It does not give you a recipe for every dish, a description of all the various dishes that were considered but not chosen or a deep analysis of why the items on the menu where selected. Naturally someone has to create the menu and a great deal of analysis might be behind every selection but this does not need to be reflected in the menu.

O’Dell has made the shortest marketing plan (here).