A brief account of how the company began- You should mention in your business plan the history behind the formation of you company and the background of the founders and directors.
Cover all bases. Before embarking on your plan it is a good idea to undertake some research in to what a professional business plan should include. You can then use this as a check list to make sure you have covered all the relevant areas. Sections of the business plan should include information on the company, the product/service market, competition in the field, management team, marketing strategy, operations and financials.
A business plan is a formal statement of business goals, reasons they are attainable, and plans for reaching them. It may also contain background information about the organization or team attempting to reach those goals.
Finally, a business plan should be error - free. This is important because your business plan defines who you are as a business person. If it turned out sloppy, then that does not speak too highly of you.