1Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
2In a large bowl, mash the sugar and margarine together until creamy, and beat in the eggs. In a separate bowl, sift the flour with baking powder and baking soda. Stir the flour mixture into the egg mixture in thirds, alternating with the mashed bananas until the batter is smooth and well combined. Gently fold in the plums, and place the batter into the prepared baking dish.
3Bake in the preheated oven until golden brown, about 1 hour.