1. Cut all but one of the plums in half and remove pits. Cut halves in half again and combine with 6 cups of water, the sugar, and the cinnamon stick in a large nonreactive saucepan.

2. Bring mixture to a boil, lower heat, and simmer, stirring occasionally, until the plums are very soft, about 35 minutes. Crush larger pieces of plums with the back of a wooden spoon. Remove from heat, remove the cinnamon stick, and stir in the lemon juice.

3. Chill thoroughly. Cut remaining plum into slices. Serve soup with scoops of sorbet and garnish with plum slices.