Wash the plums thoroughly and then dry them.
Cut the plums in half and remove the seeds. Then, finely chop the plums (you do not have to remove the skins). If using a food processor, chop all the plum halves at once on low speed and then increase to high.
In a large heat-proof bowl, combine the chopped plums, lemon juice, and sugar. Mix thoroughly and let sit for 10 minutes.
In a small saucepan, add the water and whisk in the pectin until smooth. Bring the mixture to a full rolling boil that can't be stirred down, and then boil hard for 1 minute, stirring the entire time. Remove from the heat.
Pour the hot pectin water into the bowl with the plums and stir continuously for 3 minutes.
Ladle the jam into clean freezer containers or canning jars, leaving 1/2 inch room for expansion during freezing.
Wipe the tops of the containers for any spilled jam. Then add the lids. Place the jam in the refrigerator until it has set, which may take 24 to 48 hours.
You can freeze the jam for up to 1 year or refrigerate it for 3 weeks.