Prepare the brine:
1 gallon water
3/4 cup kosher salt
1/2 cup brown sugar
1 tablespoon black peppercorns
1/2 cup grape juice
In a large pot, bring the water, salt, sugar, peppercorns, and grape juice to a boil, so that the salt and sugar dissolve. Remove the pot from heat and let the mixture cool. Chill in the refrigerator until the pot and mixture are cold. Then add to the brine mixture:
1 4-to-5-pound turkey breast
4 cups ice
Make sure the turkey breast is covered completely in the brine. Let it soak for 8 hours.
Roast the turkey:
Preheat the oven to 325 degrees. Remove the turkey from the brine, rinse it thoroughly, and blot it dry with a towel. Place the turkey breast on a large roasting pan.
2 plums, finely chopped
Ground pepper
1/4 cup butter
Slide plums under the skin of the turkey, slicing the skin if necessary to distribute across the turkey breast. Sprinkle with freshly ground pepper. Roast for 30 minutes, then rub entire breast with butter. Cover the turkey breast with foil if it browns too quickly. Roast for 1 1/2 more hours, basting with butter occasionally, until the meat registers 170 degrees on a meat thermometer. Let rest for 20 minutes before carving.