If brand is the core, and copy & design are the building blocks, a clear strategy & plan make up the foundation of your Direction and How to communicate your offer, business or service.
A communication strategy is a vision to move from where you are now, to where you want to be. It's a type of solution that deals with uncertainty because it raises the probability that you will reach your goals as outlined, but it does so mostly by creating conditions that allow for this outcome.
A strategy includes statements of intent, is purposefully unspecific, ready to be altered and improved over and over -it speaks to your overall direction.
A communication plan on the other hand, deals with the specifics that must be done. It is a programme, or arrangement with a very definite purpose. It's concrete in nature and if well crafted doesn’t allow for much deviation. At its most basic, a communication plan is a communicated course of action, aimed at reaching or exceeding a specific goal, with a predetermined timeframe.
When comparing a strategy and a plan, we see that strategy helps us understand What it is we want to achieve, and the plan lays out How we will practically go about doing so.