The basic difference here is that a firm must make its international marketing plan starting from a position of ignorance. When a firm makes a domestic marketing plan, it typically already knows a great deal about the factors that will influence its plan. It knows what sort of methods it can use to market its product, for example, and it knows what sorts of customers it is trying to reach.
By contrast, a firm entering into a new foreign market needs to gain an understanding of these and other questions. It must, therefore, ask very basic questions about itself -- it has to ask what its goals are and what kind of a firm it is, for example. It also has to discover answers to the questions mentioned above.
So an international marketing plan differs from a domestic one in that it has to start much more from the "ground up" because the firm has little knowledge of the new market.