According to the textbook, the marketing plan is a written document stating specifically the tasks to be performed to implement and control the firms marketing activities.  The marketing plan is ultimately under the strategic planning under the the marketing objectives and strategy.  I think the marketing plan describes who the firm is targeting, how they are going to do that targeting, and and if there is a good opportunity to target that market.  Many factors go into the marketing plan and can present a challenge to firms.  Some examples would be the marketing plan for the Chevy Sonic car, seems to be aimed at younger, adventurous individuals. 

http://www.youtube.com/watch?v=iuvoSw1TiJ8&seo=goo

I think the hardest part of the marketing plan is deciding who to target with your product.  How did Chevy decide that this car should target young adventurous people, and not older people?