1. Add the plums, sage, and simple syrup to the bottom of a shaker, and muddle (or smoosh vigorously with the back of a spoon) until the plums form a nice mush.
2. Fill the shaker with ice and add the lemon juice and whiskey. Shake and then double strain into a cocktail glass.
For non-alcoholic beverage, substitute in seltzer or unsweetened juice! Enjoy responsibly!