Martin dreams in 2D: in form but not in substance. By changing her famous brownies to low fat, Mrs. Brown inadvertently changes Martin's constitution as Martians are affected by polyunsaturates; basically she has poisoned him. So his dreams change from being 2D to 3D, and they also don't go away. One of his materializations is a piece of Martian seismonitricite, an explosive. Once Martin finds out what Mrs. Brown did, he tries to detoxify himself before they all get blown to smithereens.