This evening I made the fourth of these. Assuming one has a plum tree, plum pie is easy (although extremely drippy in the plum-slicing stage). These yellow plums aren't especially tart, and I use them when they're practically liquefying, so the recipe goes something like half a cup or so of tapioca and about 3/4 of a cup of sugar, onto which one slices lots of plums and occasionally stirs the mess. Somewhere along the line I start preheating the oven to 400 (that's Fahrenheit). When there seems to be about enough glop to fill the pie shell, I pour it in, and, if necessary, add a few more plums. We've tended to have blueberries in the refrigerator, so I add some of those for decoration. All the better if they're slightly dried out, because then they can absorb some of the plum juice.
I put the pan on top of a cookie sheet to minimize the drip issue, and bake for about 40-45 minutes, then drop the temperature to 350 and let it go a little longer.
The results have been well received by my writing group and the returned Parental Units.