Place plums in a heavy stockpot with water to cover. Cook until plums burst open and are tender. Remove plums, and place in a colander lined with several layers of damp cheesecloth. Gather edges of cloth up and over plums; squeeze to extract 3 cups plum juice. Discard seeds and pulp.
Return plum juice and 2 cups sugar to stock pot; bring mixture to a rolling boil, stirring frequently. Continue to boil until mixture registers 218° to 220° on candy thermometer or until jelly drops in sheets from a metal spoon. Skim off foam, and discard. Pour jelly into a hot sterilized jar, leaving 1/4-inch head space. Cover jar at once with a metal lid, and screw band tight.