1. Combine plums, sugar, and water in a large pot over medium-high heat. Bring to a boil, stirring until sugar dissolves. Lower heat to medium and cook, stirring occasionally to prevent sticking, until mixture has thickened to desired consistency, 30 to 40 minutes.

2. Meanwhile, inspect five 8-ounce jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until jelly is ready. Wash new, unused lids and rings in warm soapy water.

3. Remove jelly from the heat and skim off any heavy foam. Ladle hot jelly into hot, sterilized jars, filling to within 1/2 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw rings on finger-tight.

4. Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 15 minutes. Remove jars from water bath and tighten rings.

5. Place jars on a towel to cool until lids pop. Press lids to confirm the seal.

Cook's Note:

If you don't want to can these, cook the jelly a bit longer: check doneness by dropping a small spoonful of jelly onto a frozen plate. Let sit for 1 to 2 minutes; if jelly appears to gel, it is ready. Continue cooking if jelly appears thin and runny. When jelly has reached desired consistency, ladle into clean jars with hermetic seals. Allow to cool completely before storing in the refrigerate.