Your marketing plan is actually a statement, supported by relevant financial data, of how you are going to develop your business. Plans should be based on actions, not masses of historical data. The historical and market information should be sufficient just to explain and justify the opportunities, direction, strategy, and most importantly, the marketing actions, methods and measures - not to tell the story of the past 20 years of your particular industry.
"What you are going to sell to whom, when and how you are going to sell it, how much contribution (gross profit) the sales produce, what the marketing cost will be, and what will be the return on investment."
As stated above it is easiest and best to assemble all of this data onto a spreadsheet, which then allows data to be manipulated through the planning process, and then changed and re-projected when the trading year is under way. The spreadsheet then becomes the basis of your sales and marketing forecasting and results reporting tool.
As well as sales and marketing data, in most types of businesses it is also useful to include measurable aims concerning customer service and satisfaction.
The marketing plan will have costs that relate to a marketing budget in the overall business plan. The marketing plan will also have revenue and gross margin/profitability targets that relate to the turnover and profitability in the overall business plan. This data is essentially numerical, and so needs also some supporting narrative as to how the numbers will be achieved - the actions - but keep the narrative concise; if it extends to more than a half-dozen sheets make sure you put a succinct executive summary on the front.
The marketing plan narrative could if appropriate also refer to indirect activities such as product development, customer service, quality assurance, training etc., if significantly relevant to achieving the marketing plan aims.
Be pragmatic - marketing