Make the quick plum syrup by placing the plums, sugar, lemon juice and water in a medium skillet. Heat over medium heat to a boil, stirring frequently. Keep at a gentle boil until the plums have broken down.
To serve, unmold the icebox cake onto a serving plate. Cover the sides and top with whipped cream. Place the plated icebox cake back in the freezer for 10-15 minutes to set the whipped cream. Top with fruit and serve.