A sketch is a drawing, particularly a quick drawing that is not completely detailed. A plan can be a drawing of a building's design, showing where the rooms are, the halls are, the doors are, and so forth. So a sketch could be of a plan, but plans can be other things as well. The most common use of the word plan is to mean a sequence of actions that one has decided upon, such as, first I will have dinner, then I go for a walk, then I will watch television, then I will go to bed. That would be a fairly simple and mundane plan, but still a plan.