Combine plum juice and powdered pectin in a flat-bottomed kettle, stirring well; bring to a rolling boil, stirring constantly. Add sugar, and return to a rolling boil. Boil 1 minute, stirring constantly. Remove from heat, and skim off foam with a metal spoon.
Quickly ladle jelly into hot sterilized jars, leaving 1/4-inch headspace. Cover with metal lids, and screw bands tight. Process jelly in boiling-water bath 5 minutes.