Cream together margarine and sugar. Add eggs, flour, baking powder, and salt. Spread the batter into the bottom of a greased, square 8-inch Pyrex dish.
Arrange the plums in neat rows on top of the batter. Sprinkle with cinnamon and sugar and drizzle with one teaspoon of lemon juice.
Bake at 350 degrees Fahrenheit for 45 minutes or until golden brown.