Halve and core the plums and add sugar. Put in a tall saucepan and pour sweet red wine over them until they are just covered (so the amount of red wine depends on the amount of plums you want to process). Bring to the boil, then turn down the temperature and simmer for half an hour.
Very tasty, warm and cold. Whether plain, with ice or as an addition to a pastry (pancake, etc.).