In a sink full of cool water, rinse and de-stem the plums. Place them in a thick-bottomed pan suitable for slow cooking and deep enough to allow frothing when the plums begin to boil. Add the water and cardamom pods and bring the mixture to a low boil over medium heat. Turn heat to low for a slow simmer and allow the fruit to cook down uncovered for 1 1/2 hours. Allow the plums to cool.