1. Cut ribs into single-rib portions. Place on a roast rack set in a 2-quart glass baking dish with thick part of ribs to the outside. Cover with plastic wrap.
2. Cook in microwave oven 12 1/2 minutes at ROAST. Drain off fat. Remove rack and turn ribs over in dish.
3. Puree plums with reserved syrup. Put into a 1-quart glass measuring cup. Add orange juice concentrate and Worcestershire sauce. Cook uncovered in microwave oven 3 to 5 minutes at HIGH, or until thicker; stir once.
4. Pour sauce over ribs. Cover with waxed paper. Cook in microwave oven 10 minutes at ROAST.
5. Remove from oven. Allow to stand covered 5 minutes. Serve ribs with sauce.