For filling: Wash plums, cut them into bite sized pieces, and place them in a large bowl.
Add sugar, flour, salt, and cinnamon. Stir gently till plums are well coated. Spread into the unbaked pie shell.
For topping: In a small bowl, combine sugar, flour, cinnamon, nutmeg, and butter. Use a pastry blender, fork, or your hands to mix the ingredients together till crumbly. Sprinkle evenly over the filling.
Bake at 375 for about 50 minutes or until topping is golden brown and filling is bubbly.