Place plums, water, and lemon juice in a large non-aluminum stockpot. Stirring often, bring to a boil over medium-high heat. Cover, reduce heat to low, and simmer 10 minutes, stirring occasionally, until fruit is soft. You should end up with about 4-1/2 cups.
Stir in sugar. Return heat to high and bring back to a rolling boil while constantly stirring. Add pectin and return to a boil while stirring. Continue to stir and boil for 1 minute, then remove from heat. Let rest for 1 minute, then skim off any foam.
Pour even amounts into sterilized jars, leaving 1/8-inch of space at the top. Wipe rims and seal with sterilized lids. Process in boiling water for 5 minutes. Remove, let cool, and label jars. Store plum jam in a cool, dry place.