A marketing plan defines how a business will compete within its chosen market through its marketing strategy.
The marketing plan should be aligned with and should support the business plan.
The key element of marketing strategy within the plan that you need to compete effectively is the Segmentation, Targeting and Positioning strategy for your brand, known as STP for short. These stages in target marketing strategy development that should be central in your marketing plan are summarized in Figure 4.10 of my Digital Marketing: Strategy, Development and Planning book.
The marketing plan should also define the 7Ps of the marketing mix for a business.
For more detail, see the Smart Insights resource on how to structure a marketing plan.